How do they know when E/D committed suicide?

I've done a lot of research on Columbine and still have not yet found out how they knew that Eric and Dylan killed themselves around 12:07/12:08 PM. Did someone hear it or? How do they know?

They stopped shooting at the police at 12:05 PM. And this passage here is from "a Columbine site".

"A smoke alarm on the ceiling went off in the library at 12:08 PM, above where the shooters' bodies were later found. A small Molotov cocktail had been placed on a nearby table by one of the gunmen. The lit 'fuse' of the thing was hot enough to break the glass of the bottle, allowing the flammable liquid inside to spill out and make a small fire that set off the alarm but it didn't cause an explosion. Arson investigators determined the small fire occurred after the shooters killed themselves."

They determined it was after because they found Eric's blood and brain matter underneath the spilled liquid, meaning it was there before the liquid. So the conclusion is they killed themselves some time between 12:05 and 12:08 PM.