By Rebecca Davis O'Brien, Pervaiz Shallwani and Scott Calvert 

The 28-year-old man believed to have killed two New York City police officers as they sat in their patrol car on Saturday had a troubled emotional history and told two witnesses just before the shooting, "watch what I'm going to do," a police official said Sunday.

NYPD Chief of Detectives Robert Boyce released new details into the investigation of Ismaaiyl Brinsley, who is believed to have shot and injured his ex-girlfriend in Maryland in the morning before taking a bus to New York, where he ambushed Officers Rafael Ramos and Wenjian Liu in Brooklyn . As he was chased by police, Mr. Brinsley fatally shot himself on a subway platform.

Chief Boyce said Mr. Brinsley, who was born in Brooklyn and went to high school in New Jersey, had 19 arrests in Georgia and Ohio and was in prison from 2011 to 2013 on a weapons possession charge. Family members told police investigators Mr. Brinsley had a troubled childhood, was prone to violent outbursts, and had a history of psychiatric troubles, including at least one suicide attempt about a year ago, the chief said.

Chief Boyce said Mr. Brinsley's Facebook, Instagram and Gmail accounts and two cell phones showed anger at the government as well as "self-despair, or anger over where his life is right now." He was estranged from his two sisters, and his mother told police she was afraid of him and hadn't seen him in a month, Chief Boyce said.

There is no evidence Mr. Brinsley belonged to a gang or felt any deep religious affiliation, Chief Boyce said. The family, who are Muslim, said Mr. Brinsley never expressed any radicalized political sentiment, police said.

After shooting his ex-girlfriend, 29-year-old Shaneka Thompson, in her apartment in Owings Mills, Md., about 5:50 a.m., Mr. Brinsley called her mother to say the shooting was an accident and he hoped Ms. Thompson recovered. Baltimore County police tracked his cell phone as he traveled to New York, possibly on a Bolt bus that left him off on the West Side of Manhattan, Chief Boyce said.

Mr. Brinsley arrived in New York City at 10:50 a.m. and there is video of him taking an N train toward Brooklyn, the chief said.

Police are still trying to determine what happened in the time before the attack on the officers about 2:45 p.m. Two witnesses told police Mr. Brinsley he approached them just before the shooting, asked if they had any gang affiliation, told them to follow him on Instagram and then said, "watch what I'm going to do," Chief Boyce said.

Four shots were fired at the officers, the chief said.

Cameron McWhirter contributed to this article.

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