NYPD’s 2-hour mystery: Where was cop killer before he assassinated 2 officers?

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NEW YORK (CNN/PIX11) -- Investigators say the man who shot and killed 2 NYPD officers on Saturday was a "deeply troubled" individual who acted alone.

Mayor Bill de Blasio described his meetings Monday with the family members of the two slain New York Police Department officers.

"They are in tremendous pain," he said, "and they are worried deeply. In the Ramos family's case, two teenagers ... reminded me of my own children, who now don't have a father."

Officer Ramos' family has a strong Christian faith, de Blasio said, and his two teenage sons are "admirable, impressive young men."

Officer Liu was an only child, and had been married for just 2 months. The federal government is working with the Liu family to bring relatives from China to the U.S. to make funeral arrangements.

Online threats against police officers must be taken seriously to stop future attacks, de Blasio said, referring to social media posts by the gunman who killed the officers.

"Once this individual posted on Facebook his intention, anyone who sees that has the obligation to call the police immediately and report it," he said. "We cannot take this lightly."

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Police investigators say they're trying to piece together where the gunman who killed two officers Saturday was in the hours before the shooting.

Officials told reporters Monday there's a roughly two-hour gap in the timeline they've assembled leading up to the shooting, and they asked for the public's help to pinpoint where shooter Ismaaiyl Brinsley was before the incident.

In Baltimore, Brinsley's ex-girlfriend, 29-year-old Shaneka Nicole Thompson, was shot in the stomach but survived.

Brinsley initially put his gun to his own head, and Thompson talked him out of shooting himself, detectives said.

Investigators have also been combing through the gunman's social media posts and looking through his cell phone.

Many of his Instagram posts show "self despair," said Robert Boyce, the New York Police Department's chief of detectives. There are also anti-government tirades, he said.

Brinsley was spotted on surveillance video at the Atlantic Terminal shopping center carrying a bag. Investigators believe the gun used in the attacks was inside the bag.

Among several thousand images on Brinsley's cell phone, investigators found footage of a recent protest in New York's Union Square Park, Boyce said.

In the video, recorded around December 1, "he is a spectator watching one of the protests," Boyce said.