New York Officers’ Killer, Adrift and Ill, Had a Plan

Ismaaiyl Brinsley, who had drifted between friends and family members for most of his short life, alienating most of them and failing at almost anything that he tried, decided to come home on Saturday. He boarded a bus in Baltimore, arrived in Midtown Manhattan just before 11 a.m., and then disappeared onto the N train at the Times Square subway stop.

He was bound for Brooklyn, where he had been born 28 years before, carrying the silver Taurus 9-millimeter pistol he had used earlier to shoot his ex-girlfriend.

He had a plan, which he soon shared with the world via Instagram: He wanted to kill two police officers.

What exactly pushed Mr. Brinsley to fatally shoot two police officers before shooting himself is not clear. But by Sunday evening, several things had become obvious. He had an extensive history with the police, having been arrested 20 times, mainly for petty things, crimes ranging from stealing condoms from a Rite Aid in Ohio to shooting a stolen gun in a public street in Georgia.

Mr. Brinsley had also suffered from mental problems. Relatives told the police he had taken medication at one point, and when he was asked during an August 2011 court hearing if he had ever been a patient in a mental institution or under the care of a psychiatrist or psychologist, he said yes. He had also tried to hang himself a year ago, the police said.

By this year, Mr. Brinsley had become isolated. He was estranged from his family. His on-again, off-again relationship with Shaneka Thompson, 29, who works for the Maryland Department of Welfare and serves in the Air Force Reserve, was off again. By Saturday, he had seized on the deaths at the hands of police officers of Eric Garner on Staten Island and Michael Brown in Ferguson, Mo., focusing his rage against the authorities. In his short life, during which Mr. Brinsley failed to finish high school, to hold a steady job or, seemingly, to commit even the smallest crime without being caught, thoughts of revenge seemed to be the one thing giving him purpose.

“Most of his postings and rants are on the Instagram account, and what we’re seeing from this right now is anger against the government,” Robert K. Boyce, the Police Department’s chief of detectives, said at a news conference on Sunday. Chief Boyce added that one of those posts showed a burning flag, and in others Mr. Brinsley talked of the anger he felt toward the police. There were, Chief Boyce said, “other postings as well, of self-despair, of anger at himself and where his life is right now.”

No members of his family spoke of Mr. Brinsley with fondness. He bounced from family home to family home growing up, attending high school in New Jersey but reaching only the 10th grade. A sister in Atlanta, Nawaal Brinsley, said she had not seen him in two years. Another sister who had lived in the Bronx could not be reached, but the police said they had been called to a dispute with Mr. Brinsley at her home in 2011. Mr. Brinsley’s mother, who lives in Brooklyn, told the police she feared her son and had not seen him in a month. She said “he had a very troubled childhood and was often violent,” Chief Boyce said.

Mr. Brinsley was so transient that the police did not have a solid address for him. That made tracing his movements difficult, even as his disintegration was there for anyone to see online. But his movements on Saturday had become clearer by Sunday, according to the police.

About 5:30 a.m. Saturday, Mr. Brinsley arrived at the apartment complex of Ms. Thompson, who lives in Owings Mills, Md., just northwest of Baltimore. The two had known each other for about a year. Mr. Brinsley entered Ms. Thompson’s third-floor apartment using a key. Ms. Thompson called her mother, complaining about Mr. Brinsley’s being there. Ms. Thompson’s mother overheard the two arguing. Then the phone went dead.

A neighbor heard a woman scream, and a pop. “She was yelling, ‘You shot me, you shot me!’ ” said the neighbor, who asked to remain anonymous. “I heard him run out the door. She was yelling for help, banging, yelling: ‘Help me, help me.’ ”

The Baltimore police arrived about 5:50 a.m. Ms. Thompson then told Baltimore County police that Mr. Brinsley had shot her in the stomach and taken her cellphone, leaving his behind.

As Mr. Brinsley made his way to the Bolt Bus stop in Baltimore, he called Ms. Thompson’s mother from her daughter’s phone at about 6:05 a.m. He told her that he had shot her daughter by accident, and that he hoped that Ms. Thompson survived. (She remained hospitalized Sunday.)

By 6:30 a.m., the Baltimore County police started tracking Ms. Thompson’s cellphone. It soon pinged, moving northbound on Interstate 95. Mr. Brinsley was on the bus to New York.

As the bus traveled north, Mr. Brinsley kept calling Ms. Thompson’s mother, trying to find out Ms. Thompson’s condition.

Meanwhile, the Baltimore police were tracking his progress. By 10:49 a.m., Mr. Brinsley had arrived in New York. The phone let out a signal near 43rd Street and Eighth Avenue. A video camera caught him getting on the N train.

Once in Brooklyn, he used Ms. Thompson’s phone to make posts to Instagram. One showed a leg of his camouflage pants and his greenish shoe, spattered in blood. The other showed his pistol. “I’m Putting Wings On Pigs Today They Take 1 Of Ours...... Let’s Take 2 of Theirs #ShootThePolice,” he wrote.

At 12:07 p.m., Mr. Brinsley dropped the phone near the Barclays Center and disappeared.

The phone kept pinging, though, and the Baltimore County police contacted the police in Brooklyn. At 2:10 p.m., Baltimore County authorities reached the 70th Precinct, near where the signal had been detected, and said they had faxed over a wanted poster of Mr. Brinsley.

It was not clear if the fax was received. Police Commissioner William J. Bratton said on Saturday that it did not show up until about 2:45 p.m.

By then, time had run out. Mr. Brinsley was in Bedford-Stuyvesant. He stopped two men on a street corner. He asked them what gang they belonged to. He urged them to follow him on Instagram. Then he said they should watch what he did next.

That was when Mr. Brinsley walked past the patrol car where Officers Wenjian Liu and Rafael Ramos sat, near Myrtle and Tompkins Avenues. Crossing the street, he approached the car from behind. He fired four shots, killing both men. He fled to a nearby subway station, where he shot himself.