Warning Came Too Late in NYC Cop Killing

The accused assailant boasted about wanting to kill cops hours before his ambush.
3:18 | 12/22/14

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Transcript for Warning Came Too Late in NYC Cop Killing
We begin with the killing in new York, sending shock waves around the country. Two New York City police officers ambushed at point-blank range. The police commissioner saying they were assassinated. As other officers arrived, the street erupting into chaos. Officers following the suspect into a subway station. Riders in a subway station ordered to get down, and the grief at the hospital when the police officers died, and tonight we're learning more about the gunman and his hate-filled message before he fired. Linsey Davis is in Brooklyn, leading us off tonight. Reporter: Good evening. A massive crowd behind me. But first, we're learn ING before the shooting, the suspect approached two people on the street and said, watch what I'm about to do. Asking them to follow him on instagram. The moments of frenzy and chaos that ensued moments after two Brooklyn police officers were gunned down. A warning came, it was just too late. Police say within minutes of this ambush that killed officers wenjian liu and Rafael Ramos, two alerts came in, including a fax to the NYPD, and this wanted poster of 28-year-old ismaaiyl brinsley, the man police say is responsible for the execution-style killings. Police say it all began around 5:45 A.M. Saturday in Baltimore, Maryland. Brinsley's former girlfriend was shot and seriously wounded. Reporter: Between 1:30 and 2:00 P.M., Baltimore county detectives said instagram posts by the suspect included threats against police officers. And just one minute before the shooting, that fax coming in. Tragically, this was essentially the same time as our officers were being ambushed. Reporter: According to the NYPD, at 2:47, officers liu and Ramos were sitting in their marked patrol car in Brooklyn, when brinsley walked up to the passenger side of their vehicle and opened fire. Shots fired. Shots fired. Reporter: -- Striking both officers in the head at point-blank range. S.w.a.t. Teams chased him to a nearby subway station. Police say the suspect then shot himself in the head. Officer Ramos, officer liu died in the line of duty, protecting the city they loved. Reporter: Officers also visibly moved Saturday evening, standing at attention as the ambulance carrying the two officers passed by. Ramos joined the NYPD three years ago. He was a husband and father of two sons. On Facebook, one of his sons posting this -- today I had to say goodbye to my father, he said. He was the best father I could ask for. It's horrible that someone gets shot dead just for being a police officer. Liu had been on the job for seven years and just got married two months ago. Earlier today, there was just a handful of flowers here. Now, a massive vigil, with dozens paying respects and saying prayer for the fallen officers. Thank you. So, was the shooting death connected to the recent

This transcript has been automatically generated and may not be 100% accurate.

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