CHICAGO (CBS) — Body camera footage from one of  two Chicago police officers killed by a train is revealing new details about what the officers saw — and more importantly — what they didn’t see.

CBS 2’s Megan Hickey takes a closer look the officers who lost their lives and the events leading up to their last moments.

After reviewing body cam video from the scene, police said that the officers were looking in the direction of an oncoming train and did not see or hear the train that was coming from behind them, taking their lives within an instant.

At 6:15 Monday night, a report of gunfire, thanks to a hi-tech sound detector called a ShotSpotter. Thirty seconds later, a unit is dispatched. They must have been close because they’re on the scene seconds later.

Three minutes later, dispatchers receive word that someone is running south on the east side of the tracks in Rosemoor.  Two police officers, Conrad Gary and Eduardo Marmolejo went west up the embankment to the tracks, searching for a a gunman.

From looking at body cam video, police said the officers then went south on the tracks, west of a train that was headed inbound.

All of this happened just as the outbound South Shore Line train was approaching from the north and an inbound train was approaching from the south.

Police believe that sound drowned out the noise of the outbound train behind them.

They never saw it.

The train operator slammed on the brakes but the train was going some 60 miles an hour and the officers died instantly near 103rd and Cottage Grove.

“It was just crazy,” said passenger Greg Brewer. “I mean it happened so quick.”

By 6:21 p.m., a dispatcher said that the suspect is in custody. About 10 minutes later, dispatch reports a call from police that a man had been hit.

Several tense minutes pass before it became clear that there were two people hit and that they were police officers, killed in the line of duty while trying to serve and protect the city of Chicago.

Sources said a suspect is in his 20s and had apparently never been arrested. Allegedly he found the gun, was testing it out and fired it to see if it worked. He was interrogated on Tuesday and charges could be coming soon.