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

Context articles: 2009 shooting of Pittsburgh police officers
Navajo Nation Police
Law enforcement officer
Mark Hobson
Dale Cregan
Association of Chief Police Officers
May 2009 Dera Ismail Khan bombings
Murders of Nicola Hughes and Fiona Bone
Howard Morgan case
Frank Jude, Jr.

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.
The Navajo Nation Police (formerly known as the Navajo Tribal Police) is the law enforcement agency on the Navajo Nation in the Southwestern United States.
 It is under the Navajo Division of Public Safety.
 It is headed by a Chief of Police, six Police Captains and eight Police Lieutenants.
 It includes: Internal Affairs, Patrol, K-9 Unit, Police diving, Tactical Operations Team, Traffic Unit, Fiscal management, Recruitment, and Training Divisions.
 The Navajo Nation Police are responsible for seven districts: Chinle, Crownpoint, Dilkon, Kayenta, Shiprock, Tuba City, and Window Rock.
 There are also several substations in each district ranging from one-man substations or up to five officers each.
 Currently, there are 210 sworn police officers(134 patrol), 28 criminal investigators and 279 civilians, acting as support staff for the department .
 There are approximately 1.9% police officers per 1,000 people and one officer is responsible for patrolling 70 sqmi of reservation land.
 The Navajo Nation Police are funded by federal contracts and grants and general Navajo Nation funds.
 This police department is one of only two large Native American police Departments with 100 or more sworn officers in the United States (the other is the Oglala Lakota Nation's police department).
A law enforcement officer (LEO) or peace officer, in North American English, is a public-sector employee whose duties primarily involve the enforcement of laws.
 The phrase can include police officers, municipal law enforcement officers, special police officers, customs officers, state troopers, special agents, special investigators, border patrol officers, immigration officers, court officers, probation officers, parole officers, arson investigators, auxiliary officers, game wardens, sheriffs, constables, corrections, marshals, deputies, and Public Safety Officers (at public institutions).
 Security guards are civilians and therefore not law enforcement officers, unless they have been granted powers to enforce particular laws, such as those accredited under a Community Safety Accreditation Scheme.
 Although typically the term "law enforcement officer" refers to those government agents with police powers, prosecutors are also law enforcement officers.
Mark Richard Hobson (born 2 September 1969) is a British spree killer who killed four people in North Yorkshire, England in July 2004.
 He was arrested after an eight-day nationwide manhunt involving more than 500 police officers and 12 police forces, during which time he was Britain's "most wanted man".
 Police discovered notes written by Hobson that showed the murders were pre-meditated and well-planned, including a 'to-do' note detailing how he planned to lure his girlfriend's twin sister to his flat and a shopping list for "big bin liners," tape, tie-wraps, fly spray and air freshener.
 Against his girlfriend's sister's name he had written "use and abuse at will."
 The list of planned victims also included his girlfriend's parents and the parents of his ex-wife.
 Hobson was tried for the murders in April 2005.
 Pleading guilty, he was sentenced to four terms of life imprisonment with a recommendation that he should never be released.
 This was one of the first times that such a recommendation had been made for someone who had admitted their crime at the first opportunity.
 He is currently incarcerated at Wakefield Prison, West Yorkshire.
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.
The Association of Chief Police Officers (ACPO), officially The Association of Chief Police Officers of England, Wales and Northern Ireland, was a not-for-profit private limited company that for many years led the development of policing practices in England, Wales, and Northern Ireland.
 Established in 1948, ACPO provided a forum for chief police officers to share ideas and coordinate their strategic operational responses, and advised government in matters such as terrorist attacks and civil emergencies.
 ACPO coordinated national police operations, major investigations, cross-border policing, and joint law enforcement.
 ACPO designated Senior Investigative Officers for major investigations and appointed officers to head ACPO units specialising in various areas of policing and crime reduction.
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.
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.
The Howard Morgan case revolves around an incident that took place on February 21, 2005 in Chicago, Illinois.
 Howard Morgan, a retired officer of the Chicago Police Department, was shot 28 times by four active Chicago police officers: John Wrigley, Eric White, Timothy Finley and Nicolas Olsen.
 Morgan was accused of aggravated battery, discharging a firearm, and attempted murder; no charges were filed against the police officers.
 Morgan was found not guilty of battery or discharging a firearm in his 2007 trial; no verdict was returned on attempted murder.
 Morgan was tried a second time for attempted murder and convicted in January 2012.
 In April 2012, he was sentenced to 40 years in prison.
 His case drew outrage from activists within Chicago and around the country.
 In January 2015, departing Illinois governor Pat Quinn granted him clemency.
Frank Jude, Jr., a.k.a. Frankie Lee Jude, Jr., (born August 14, 1978) is a Wisconsin man who was severely beaten by off-duty Milwaukee police officers in the early-morning hours of October 24, 2004.
 Following a state trial that ended with the jury acquitting the three police officers charged, a federal investigation led to plea agreements with three police officers and the indictment of five police officers, including the three who were acquitted in state court.
 Before trial, one of these five pleaded guilty.
 The federal jury acquitted one of the remaining police officers and the three police officers who were acquitted in state court were convicted in federal court.