(CNN) -- The day ended with two slain New York police officers. But it started, investigators say, when Ismaaiyl Brinsley slipped into his ex-girlfriend's apartment using a key he wasn't supposed to have.
A fight quickly erupted around 5:30 a.m. Saturday after Brinsley used that key to unlock the door of an apartment in the Baltimore suburb of Owings Mills, Maryland, New York Police Chief of Detectives Robert Boyce told reporters.
Just 20 minutes later, calls of a shot fired there came into 911.
Shaneka Nicole Thompson, a 29-year-old who'd known Brinsley for about a year and was once romantically involved with him, had been shot in the stomach with a 9mm semiautomatic handgun.
By the time police arrived, police said, Brinsley had fled -- stealing Thompson's cell phone and carrying it with him as he headed north on the highway.
Minutes later, he called her mother and apologized, telling her he'd shot Thompson "by accident and that he hopes she lives," Boyce said.
She was critically wounded, police said, but is expected to survive.
It was one of Thompson's friends, Baltimore County Police said, who alerted them Saturday afternoon to troubling Instagram posts the friend believed came from Brinsley.
"These posts included overt threats to kill police officers," police said, and they appeared to be posted in Brooklyn, New York.