Question: The murders of which police officers led to Dale Cregan being sentenced to life in prison?

full_context: Document 1: Dennis Nilsen: Dennis Andrew Nilsen (born 23 November 1945) is a Scottish serial killer and necrophiliac, also known as the Muswell Hill Murderer and the Kindly Killer, who murdered at least 12 young men in a series of killings committed between 1978 and 1983 in London, England. Convicted of six counts of murder and two of attempted murder at the Old Bailey, Nilsen was sentenced to life imprisonment on 4 November 1983, with a recommendation that he serve a minimum of 25 years. He is currently incarcerated at HMP Full Sutton maximum security prison in Full Sutton in the East Riding of Yorkshire, England.
Document 2: Dale Cregan: Dale Christopher Cregan (born 6 June 1983) is an English convicted drug-dealer and murderer who was sentenced to a whole life order in prison for four counts of murder (including the killing of two police officers) and three separate counts of attempted murder, meaning that he will never be released from prison.
Document 3: Cumbria shootings: The Cumbria shootings was a killing spree that occurred on 2 June 2010 when a lone gunman, Derrick Bird, killed 12 people and injured 11 others before killing himself in Cumbria, England. Along with the 1987 Hungerford massacre, the 1989 Monkseaton shootings, and the 1996 Dunblane school massacre, it is one of the worst criminal acts involving firearms in British history.
Document 4: Corporals killings: British Army corporals David Howes and Derek Wood were killed by the Provisional IRA on 19 March 1988 in Belfast, Northern Ireland, in an event which became known as the corporals killings. The off-duty soldiers were wearing plain-clothes when they were killed by the IRA after encountering the funeral procession of an IRA member whilst returning from Belfast to Lisburn. Three days before, loyalist paramilitary Michael Stone had attacked an IRA funeral and killed three people. Believing the soldiers were loyalists intent on repeating Stone's attack, dozens of people surrounded and attacked their car. During this, Corporal Wood drew his service pistol and fired a shot in the air. The soldiers were then dragged from the car, beaten, and taken to nearby waste ground where they were stripped and shot dead.
Document 5: David Bieber: David Francis Bieber (born 3 February 1966) also known under the alias Nathan Wayne Coleman is an American convicted murderer. A fugitive from the United States, he murdered PC Ian Broadhurst and attempted to murder PCs Neil Roper and James Banks on 26 December 2003 in Leeds, England, sparking a nationwide search before he was captured. He was given a whole life sentence after being found guilty of murder in December 2004 and the trial judge recommended that he should never be released, however, in 2008 this sentence was controversially reduced to a minimum term of 37 years by the court of appeal, after which he could apply for parole.
Document 6: Grim Sleeper: Grim Sleeper is the nickname for convicted serial killer Lonnie David Franklin Jr., responsible for at least ten murders and one attempted murder in Los Angeles, California. The attacker was dubbed the "Grim Sleeper" because he appeared to have taken a 14-year break from his crimes, from 1988 to 2002. In July 2010, Franklin was arrested as a suspect, and, after many delays, his trial began in February 2016. On May 5, 2016, the jury convicted him of killing nine women and one teenage girl. On June 6, 2016, the jury recommended the death sentence, and on August 10, 2016, Los Angeles Superior Court sentenced him to death for each of the ten victims named in the verdict.
Document 7: Andrew Chan: Andrew Chan (; 12 January 1984 – 29 April 2015) was an Australian drug smuggler who was convicted and executed in Indonesia for drug trafficking as a member of the Bali Nine. In 2005, Chan was arrested at Ngurah Rai International Airport in Denpasar. According to court testimonies of convicted drug mules, Chan and Myuran Sukumaran were the co-ringleaders of the heroin smuggling operation from Indonesia to Australia. After a criminal trial on 14 February 2006, Chan was sentenced to execution by firing squad by the Denpasar District Court.
Document 8: New Lodge Six shooting: In the late hours of 3 February and the early hours of 4 February 1973, six men, all of whom were Catholics, were shot and killed in the New Lodge area of north Belfast, four (one IRA member and three civilians) of them were shot dead at the junction at Edlingham Street by British Army snipers, the other two men were shot dead by the Ulster Defence Association (UDA). Irish Nationalists and Republicans believe that collusion took place between the British security forces and Loyalist paramilitaries in the shootings. The six men who were killed became known as "The New Lodge Six".
Document 9: 2009 shooting of Pittsburgh police officers: On April 4, 2009, a shootout occurred at 1016 Fairfield Street in the Stanton Heights neighborhood of Pittsburgh, Pennsylvania, United States, stemming from a mother and her 22-year-old son's argument over a dog urinating in the house. At approximately 7:11 a.m. EDT, 22-year-old Richard Poplawski opened fire on two Pittsburgh Police officers responding to a 9-1-1 call from Poplawski's mother, who was attempting to get the police officers to remove her son from the home. Despite Poplawski's mother telling the 9-1-1 operator that Poplawski had guns, the police officers were not told. Three police officers were ultimately confirmed dead, and another two were seriously injured.
Document 10: Allan Grimson: Allan Grimson (born 1958) is a convicted British murderer who is responsible for murdering at least two men and is suspected of killing others, possibly up to another 20 undiscovered victims. The judge, who sentenced him to a minimum term of 22 years at his trial, said that Grimson was a serial killer by nature, but not by number. Because his two victims were killed on the same date just a year apart (12 December), detectives believe there may be more victims out there as yet unidentified.
Document 11: May 2009 Dera Ismail Khan bombings: On May 14, 2009, three Hand Grenades were thrown on Police officers in Dera Ismail Khan. A motorcycle rider threw a hand grenade on police emergency office. According to local Police Chief of city, Muhammad Iqbal, 5 police officers were wounded in first grenade's explosion three of whom were seriously hurt. In the next half an hour, two more grenades were lobbed on two police posts near a bus stand. Four police officers were injured in next two explosions. In all the three explosions, 9 police offers were injured. All injures were brought to a state–run hospital. Ashiq Saleem, doctor at hospital, said, "we have received eight injures, three of them are serious, all of them are police officers." No group have yet claimed responsibility for attacks.
Document 12: Christopher Dale Flannery: Christopher Dale Flannery, nicknamed Mr. Rent-a-Kill (born 1948 – missing and believed murdered since 9 May 1985) is alleged to have been an Australian contract killer. Flannery was born in Brunswick, Victoria.
Document 13: Shepherd's Bush murders: The Shepherd's Bush murders, also known as the Massacre of Braybrook Street, involved the murder of three police officers in London by Harry Roberts and two others in 1966.
Document 14: Paul Grimes (criminal): Paul Grimes (born 26 May 1950) is a former gangster who, from an early age, was active in Liverpool's criminal underworld. He has 38 criminal convictions and was involved in a range of violent and illegal activities. He also set up legal businesses recycling scrap metal and disposing of waste. He was rich, successful and at the top of the gangster hierarchy when his son Jason died of a heroin overdose in 1992, at the age of 21. This tragedy led to Grimes becoming a police informer with the aim of bringing down the drug dealers who he felt had destroyed his son's life. His evidence has led to successful prosecutions against high-profile dealers such as John Haase and Curtis Warren. The information Grimes provided also led to his son Heath being jailed for five years.
Document 15: Murders of Nicola Hughes and Fiona Bone: On 18 September 2012, Police Constables Nicola Hughes and Fiona Bone, two Greater Manchester Police officers, were killed by Dale Cregan in a gun and grenade ambush while responding to a report of a burglary in Greater Manchester, England.
ground_truth: Nicola Hughes and Fiona Bone