The scene of the shooting ... Two police officers were killed, execution-style in Brooklyn, New York City. Picture: William Farrington Source: New York Post

Investigators are probing a possible connection between shooter Ismaaiyl Brinsley and the Baltimore prison gang who have declared open season on cops in revenge for the deaths of Eric Garner and Mike Brown, African American men who were killed by white police officers who escaped murder charges.

Police say Brinsley murdered his ex-girlfriend early in the morning in Baltimore, the largest city in the State of Maryland, the largest independent city in the United States, before posting of his plans to kill cops and travelling to New York, where he shot the two police officers execution-style.

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“They take 1 of ours ... I take 2 of theirs” ... Ismaaiyl Abdullah Brinsley who executed two NYPD police officers after posting the comments on his Instagram account. Source: Facebook

Shot point blank ... Two uniformed NYPD officers, Rafael Ramos and Wenjian Liu, were shot dead on Saturday, December 20, while sitting in their marked police car on a Brooklyn street corner. Source: Huffington Post

Detectives have travelled to Baltimore to investigate Binsley’s link to the gang, reports The New York Daily News.

The gang was started in the 1960s by Black Panther George Jackson in a California prison, the newspaper reports. The radical American black nationalist group the Black Panthers began as a movement against racist police brutality in the late 1960s.

The gang’s calls for revenge come as the US has been hit by mass protests after grand juries failed to bring officers to trial after the deaths of Eric Garner and Mike Brown.

An undercover NYPD officer learned of a Black Guerilla Family plan to kill a NYPD cop on Dec. 5, with at least ten gang members ready to shoot police officers.

Officers were advised by their union to carry extra ammo and wear bulletproof vests, but the NYPD later said there was no real threat.

Brinsley bragged about his plans, posting photos of a gun to social media before the attack, writing that he was “putting wings on pigs today.”

Tragically, his girlfriend’s family and Baltimore police tried to warn the NYPD, but were too late.

New York City’s Police Commissioner William Bratton said officers Rafael Ramos and Wenjian Liu “were, quite simply, assassinated.”

“Today, two of New York’s finest were shot and killed with no warning, no provocation,” he said.

Ramos and Liu were working overtime as part of an anti-terrorism drill when they were shot point-blank in their heads by Ismaaiyl Brinsley, who approached them on foot from the sidewalk at the corner of Myrtle and Tompkins avenues in Bedford-Stuyvesant, Brooklyn.

Moments after killing the two officers, 28-year-old Brinsley, who had addresses in Georgia and Brooklyn, was dead, having turned the gun on himself on a nearby subway platform as cops closed in.

“I’m Putting Wings on Pigs Today,” a person believed to be the gunman wrote on Instagram in a message posted just three hours before the officers were shot through their front passenger window.

Instagram posting ... Two hours before two NYPD officers were shot dead, execution-style in Brooklyn. Picture: Instagram Source: Instagram

The post included an image of silver automatic handgun with a wooden handle. Another post showed camouflage pants and blue sneakers which matched the clothing the dead gunman was wearing as his body was carried from the scene on a stretcher.

“They Take 1 Of Ours … Let’s Take 2 of Theirs,” the post continued, signing off with, “This May Be My Final Post.”

Wore to the killing of two police officers ... Image posted on Instagram hours before the shooting of Ismaaiyl Brinsley's camouflage pants and distinctive blue sneakers. Picture: Instagram Source: Instagram

He used the hashtag #ShootThePolice, along with two other hashtags Garner and Brown.

Brinsley walked up to the cops’ patrol car at the corner of Myrtle and Tompkins avenues, approaching from the sidewalk.

Witnesses told police that Brins­ley wordlessly blasted into the patrol car’s front passenger-side window.

Then he stood stock still for a few moments, fleeing into the nearby subway station, the G-train station at Myrtle and Willoughby avenues, where, as pursuing cops closed in, he shot himself on a crowded platform, sources told The Post.

“They engaged the guy and he did himself,” one investigator said of the gunman’s demise.

Both shooting scenes — above and below ground — were scenes of blood and terror.

“I heard shooting, — four or five shots,” eyewitness Derrick McKie, 49, told The Post of the cops’ tragic murder. “It sounded like from a single gun,” he said. Ambulances and police cars rushed to the scene, he said.

“I seen them putting the cop in the ambulance. He looked messed up,” McKie, a barber, added. “He took a high calibre weapon to the face. He was lifeless … I couldn’t see where the holes was that, all I could see was blood. His body was lifeless.”

Claimed he was taking revenge for the killing of Eric Garner and Michael Brown ... Gang member Ismaaiyl Brinsley shot dead two policeman as they sat in their marked car. Picture: Facebook Source: Facebook

"This May Be My Final Post. I'm Putting Pigs In A Blanket" ... The crazed gunman Ismaaiyl Brinsley who shot dead the two NYPD officers sitting in their marked car after earlier murdering his girlfriend. Picture: Facebook Source: Facebook

US President Barack Obama “unconditionally” condemned the murder of the two police officers, saying there was no justification for the killings.

Police offers risk their own safety to serve and protect their communities and that they deserve the public’s respect and gratitude, he said.

Mr Obama asked Americans to reject violence and harmful words, and encouraged people to embrace words that heal, and to seek out prayer and sympathy for the victims’ relatives.

U.S. Attorney General Eric Holder called the shooting deaths an “unspeakable act of barbarism.”

Modal Trigg Carmen Jimenez, 32, a social worker from Bedford-Stuyvesant, was on the subway platform when the gunman ran inside, pursued by officers.

“Everything happened so quick,” said Jimenez, who is eight months pregnant. “We were standing waiting for the G train. We heard arguing from the other end of the platform.

It looked like two cops came in there was lots of yelling and they said, ‘Everybody get down.’

“We tried to get out of there, and there was a lot of shouting, people were screaming, people were trying to run.

“I threw myself on the floor. I was afraid for my life and afraid for my baby.”

The two officers were pronounced dead at Woodhull Hospital, where their colleagues and family members huddled tearfully.

City Council President Melissa Mark-Viverito and Mayor Bill de Blasio were less than welcome guests at the poignant gathering.

Mr DeBlasio has been criticised by police for his support of anti-cop protesters, and was previously asked not to attend any police funerals.

“We’re all in this together,” the mayor told grieving cops, according to a cop who was there.

“No we’re not,” one officer said tersely in response.

Mayor’s tears. NYPD Commissioner Bill Bratton, right, stands beside Mayor Bill de Blasio as he wipes his eye during a news conference at Woodhull Medical Centre. Picture: AP Source: AP

The Rev. Al Sharpton, a civil rights activist, said that Garner’s family had no connection to the suspect and denounced the violence.

“Any use of the names of Eric Garner and Michael Brown in connection with any violence or killing of police, is reprehensible and against the pursuit of justice in both cases,” Sharpton said.

Demonstrators around the US have protested since a grand jury decided on December 3 not to indict the officer involved in Garner’s death, a decision that closely followed a Missouri grand jury’s decision not to indict an Officer Darren Wilson in the fatal shooting of 18-year-old Brown.

Several New York officers were assaulted during mass demonstrations last weekend, including one that drew thousands to the Brooklyn Bridge.

Investigators work at the scene where two NYPD officers were shot and killed in the Bedford-Stuyvesant neighbourhood of the Brooklyn borough of New York. AP / John Minchillo Source: AP

The president of the police officers union, Patrick Lynch, and Mayor Bill de Blasio have been locked in a public battle over treatment of officers following the decision not to indict the officer in Garner’s death. Just days ago, Lynch suggesting police officers sign a petition that demanded the mayor not attend their funerals should they die on the job.

The last shooting death of an NYPD officer came in December 2011, when 22-year veteran Peter Figoski responded to a report of a break-in at a Brooklyn apartment. He was shot in the face and killed by one of the suspects hiding in a side room when officers arrived. The triggerman, Lamont Pride, was convicted of murder and sentenced in 2013 to 45 years to life in prison.