Police are investigating whether the 28-year-old man who shot two US patrolmen dead in New York had links to a notorious gang that recently vowed to kill police officers.
Ismaaiyl Brinsley shot and wounded his ex-girlfriend before travelling to Brooklyn to kill the officers.
New York City's police commissioner Bill Bratton choked up at a news conference as he talked about the murders. He said the gunman who ambushed and killed Rafael Ramos and Wenjian Liu had made online posts that were "very anti-police".
Brinsley apparently posted them on his ex-girlfriend's social media account after he shot her in the stomach shortly before 6am at her apartment in Baltimore. Officials later said 29-year-old Shaneka Nicole Thompson was in a critical but stable condition.
In the Instagram messages, Brinsley had said he was "putting wings on pigs". He then referred to Eric Garner and Michael Brown, two unarmed black men whose deaths at the hands of police have sparked national protests, before adding: "Let's take two of theirs."
As tensions between minorities and police remain high across the US, another officer was shot and killed in Tarpon Springs, Florida yesterday. A suspect has been taken into custody.
Police said Brinsley approached the passenger window of a patrol car on Saturday and opened fire, striking officers Ramos and Liu in the head. "They were, quite simply, assassinated - targeted for their uniform," Mr Bratton said.
President Barack Obama, holidaying in Hawaii, condemned the killings, saying: "The officers who serve and protect our communities risk their own safety for ours every single day, and they deserve our respect and gratitude every single day."
Some New York media suggested Brinsley may have been a member of the Black Guerrilla Family (BGF), a gang originally formed inside Californian jails in the late 1960s and which has more recently been active in Baltimore.
Earlier this month, it was reported the BGF had vowed to kill police officers.The Baltimore Sun stated that a federal law enforcement official had said Brinsley had no known ties to the BGF.