5.  The applicants, Mr K.S. and his wife Mrs M.S., were born in 1939 and 1942 respectively and live in Lauf.
6.  In 2006 the German secret service (Bundesnachrichtendienst) bought a data carrier from a certain K. for a considerable amount of money. The data carrier contained financial data from the Liechtenstein L. Bank relating to 800 people. K., who had formerly been an employee of the L. Bank, had illegally copied the data. The data carrier was submitted to the German tax investigation authorities, which subsequently instigated proceedings against, inter alia, the applicants, in relation to tax evasion crimes.
7.  On 10 April 2008 the Bochum District Court (“the District Court”), following an application from the Bochum prosecutor’s office, issued a search warrant in respect of the home of the applicants, who were suspected of having committed tax evasion between 2002 and 2006. The search warrant allowed the seizure of papers and other documents concerning the applicants’ capital, both inside and outside Germany, especially documents concerning information on foundations and any documents that could help to determine the true tax liability of the applicants since 2002.
8.  The search warrant indicated that, in the course of investigations against another suspect, the prosecution had obtained information that the applicants had established the “K. Foundation” on 17 January 2000 and the “T.U. S.A.” on 14 June 2000. The applicants were suspected of having made financial investments via these two associations with the L. Bank in Liechtenstein, for which they were liable for tax in Germany. According to the search warrant, the applicants had failed to declare about 50,000 euros (EUR) of the yearly interest accrued from the capital of both the K. Foundation and T.U. S.A. in their tax returns for the years 2002 to 2006. It indicated that the applicants had evaded tax payments of EUR 16,360 in 2002, EUR 24,270 in 2003, EUR 22,500 in 2004, EUR 18,512 in 2005 and EUR 18,000 in 2006. The search warrant stated that the house search was urgently needed in order to find further evidence and that, weighing the seriousness of the alleged crimes against the constitutional rights of the applicants, the house search was proportionate.
9.  On 23 September 2008 the applicants’ flat was searched and one envelope containing L. Bank documents and five computer files were seized.
10.  The applicants appealed against the search warrant. They argued that the warrant had not been granted in accordance with the law. It had been based on material which had been acquired in breach of international law, especially the European Convention on Mutual Assistance in Criminal Matters of 20 April 1959 and the Convention on Laundering, Search, Seizure and Confiscation of the Proceeds from Crime of 8 November 1990, because the data had been stolen from the L. Bank and had been bought by the secret service. The acquisition of the data had also violated domestic law, as the secret service had no authorisation to obtain tax data. In fact, such an act was a criminal offence under German law, as it infringed section 17(1) and section 17(2)(2) of the Unfair Competition Act (“divulgence of official secrets” (Geheimnisverrat)). Furthermore, the secret service was not authorised to forward tax data to the financial authorities and the prosecution, as this infringed the German legal principle of separation of the secret service and the police/prosecution (Trennungsprinzip).
11.  On 8 April 2009 the Bochum District Court dismissed the appeal. It was of the view that the house search had been legal as it had been based on a lawful search warrant. The court had no doubt that it had been lawful to base the search warrant on the information contained in the Liechtenstein data carrier, as, in its view, the data had neither been seized in direct violation of international law nor by circumventing international treaties.
12.  The District Court was also of the view that the search warrant in question could be issued on the basis of the information in the data carrier, as the secret service had only played a passive role in acquiring it. According to the court, there was no indication that the secret service had incited a third person to steal the data, and it had merely accepted the data from a third person when this person had offered it. The fact that the secret service might have remunerated the seller did not, in the court’s view, change the fact that the secret service had only played a passive role. In the court’s opinion, it was within the secret service’s remit to acquire the data carrier in the prescribed way and hand the data over to the prosecution, as the data carrier contained 9,600 sets of data concerning international cash flows.
13.  On 7 August 2009 the Bochum Regional Court dismissed the applicants’ appeal. It held that the search warrant had been lawful, even if it was true that the German authorities had infringed domestic criminal law in obtaining the evidence. Even assuming that the German authorities might have committed the criminal offences of acting as an “accessory to a criminal offence” (Begünstigung, Article 257 § 1 of the German Criminal Code) and an “accessory to the divulgence of official secrets” (Beihilfe zum Geheimnisverrat, section 17(1) and section 17(2)(2) of the Unfair Competition Act, in conjunction with Article 27 of the German Criminal Code) in buying the Liechtenstein data from K., and that K. might have committed the offence of “industrial espionage” (Betriebsspionage, section 17(2)(1) of the Unfair Competition Act), it considered the search warrant to have been lawful. With regard to the applicants’ allegation that the data had been acquired in breach of international law, the Regional Court doubted any such breach.
14.  As regards the question whether illegally obtained evidence could be used in criminal proceedings, the Regional Court referred to a decision of the same court of 22 April 2008, where it had held in a similar case and with regard to the same data carrier that the interest in prosecuting the suspects outweighed the possible infringements of criminal law, as the principal criminal act of “data theft” had been committed by a third party and not by the German authorities. According to the well-established case-law of the Federal Court of Justice, evidence that had been illegally acquired by a third party could generally be used in criminal proceedings, unless it had been acquired through coercion or force. It also had to be considered that the use of the “stolen” data had not infringed the core of the applicants’ private sphere, but their business affairs. Furthermore, the “data theft” had not primarily infringed the rights of the applicants, but the data-protection rights of the bank from which it had been “stolen”. Thus, the Liechtenstein data was not excluded as evidence and the search order could be based on it. As to the presumed breach of international law, the court added that such a breach would not lead to the unlawfulness of the search warrant, firstly because international law did not grant the applicants any personal rights and secondly because the use of the evidence did not in itself constitute a breach of international law.
15.  On 11 September 2009 the applicants lodged a constitutional complaint with the Federal Constitutional Court. They were of the view that the Regional Court and the District Court should have decided that the search warrant had not been in accordance with the law, as the use of the Liechtenstein data as a basis for a search warrant had violated international treaties and the sovereignty of Liechtenstein, which had protested against the use of the data.
16.  Furthermore, they argued that their right to respect for their home under Article 13 of the Basic Law had been infringed, as the search warrant had been based on evidence that had been acquired by the secret service and passed on to the prosecution in violation of domestic law. The data purchase from K. had constituted a criminal act. Moreover, the secret service had no authority under German law to purchase such data. Furthermore, the transfer of the Liechtenstein data from the secret service to the financial authorities and the prosecution had violated the principle of the separation of the secret service and the prosecution in Germany. The infringement of domestic law had been so severe that the criminal courts should have come to the conclusion that the Liechtenstein data could not have formed the basis of a search warrant. They would thus have been obliged to declare the search warrant illegal.
17.  On 9 November 2010 the Federal Constitutional Court dismissed the constitutional complaint as manifestly ill-founded. It found that the fact that the search warrant had been based on the Liechtenstein data did not infringe Article 13 of the Basic Law.
18.  The Federal Constitutional Court reiterated that there was no absolute rule that evidence which had been acquired in violation of procedural rules could never be used in criminal proceedings (compare paragraph 28 below). The court further pointed out that it had to be borne in mind that the case at hand did not concern the question of whether evidence could be admitted in a criminal trial, but only concerned the preliminary question of whether evidence that might have been acquired in breach of procedural rules could form the basis of a search warrant in criminal investigation proceedings. Even if evidence was considered inadmissible in criminal proceedings, this did not automatically mean that the same was true for all stages of criminal investigations.
19.  Furthermore, the court reiterated that it was not its role to substitute itself for the authorities in the interpretation and application of domestic law, but to review, in the light of the Basic Law, the decisions taken by the authorities in the exercise of their margin of appreciation.
20.  In applying these general principles to the case at hand, the Federal Constitutional Court ruled, at the outset, that it was not necessary to decide upon the question whether the acquisition of the data had been in breach of national or international law or violated the principle of the separation of the secret service and the prosecution in Germany, as the Regional Court had departed in its decision from the applicants’ allegation that the evidence might in fact have been acquired in breach of domestic and international law, including criminal law.
21.  The Federal Constitutional Court found that the fact that the Regional Court based its legal assessment on the assumption that the acquisition of the data had been in breach of domestic and/or international law was not arbitrary and hence could not be found to be in violation of Article 13 of the Basic Law. Its finding that the applicants could not invoke international law in their favour only showed a different legal opinion without disregarding the applicants’ basic rights. Furthermore, the Federal Constitutional Court considered reasonable the District and Regional Court’s legal assessment that the principle of separation of the secret service and the prosecution had not been infringed, as the facts of the case did not show that the secret service had either ordered or coordinated the “data theft”, but had been offered the data on K.’s own initiative. Acquiring data in such a way and passing it on to the prosecution could not violate the principle of separation, and hence could not render a search warrant unconstitutional.
22.  With regard to the Regional Court’s finding that the search order could be based on the Liechtenstein data the Federal Constitutional Court found that the Regional Court’s legal assessment sufficiently took into account the applicants’ basic rights as the Regional Court had departed from the applicants’ allegation that the evidence had been obtained in breach of domestic law and thus based its decision on, what was, for the applicants, the best possible assumption.
23.  The Federal Constitutional Court further considered that the Regional Court had struck a fair balance between the different interests at stake. The alleged breach of national and/or international law did not entail an imperative prohibition to use the evidence in the proceedings at issue. Furthermore, the Regional Court had rightly pointed out that the data did not relate to the core area of the applicants’ private life, but to their business activities. It had recognized the decisive interest at stake, namely the applicants’ right to inviolability of their home, and took it sufficiently into account, as nothing showed that German authorities purposely and systematically breached international or domestic law in obtaining the data carrier.
24.  On 2 August 2012 the Nuremberg District Court acquitted the applicants of the charges of tax evasion, finding that it had not been proven beyond reasonable doubt that the capital of the foundation in question had been invested in an interest-bearing way.