In August 1968, the United States military sent a Star III mini-submarine to the base to look for weapon debris, especially the uranium-235 fissile core of a secondary. A much bigger operation at Palomares off the coast of Spain two years earlier led to the recovery of a lost nuclear weapon from the Mediterranean Sea; the B28FI bomb was lost for 80 days after a mid-air collision between a B-52 on a "Chrome Dome" mission and its KC-135 Stratotanker refueling aircraft. Christensen asserts that the purpose of the underwater search at Thule was obvious to the Danish authorities, contrary to other reports that suggested its true purpose had been hidden from them. At lower levels, however, the dives were surrounded by some confidentiality. One document from July 1968 reads, "Fact that this operation includes search for object or missing weapon part is to be treated as Confidential NOFORN", meaning it was not to be disclosed to non-US nationals. It continues, "For discussion with Danes, this operation should be referred to as a survey, repeat survey of bottom under impact point." Further indications of the search are apparent in a September 1968 interim report by the United States Atomic Energy Commission, which stated, "It was further speculated that the missing "&lt;redacted&gt;", in view of its ballistic characteristics, may have come to rest beyond the observed concentration of heavy debris." This discussion was a reference to the unsuccessful search for the uranium cylinder of one of the secondaries.