Lea Michele is reaching out to friends of her late love Cory Monteith, trying to learn exactly what triggered his relapse into drug abuse that led to his fatal overdose of alcohol and heroin July 13.

"Lea is trying to talk to the people who last saw him," an insider tells US Weekly. "She wants to know what happened, she wants to understand."

As we previously reported, when Monteith entered a rehab facility this past spring, Lea stood behind her boyfriend, saying, “I love and support Cory and will stand by him through this and I am grateful and proud he made this decision.”

A source told the magazine that "Lea long knew about Cory's drug problems and was always pushing him to get help, as she believed he'd be unstoppable if he could beat his demons."

Lea, who dated Cory for 14 months, "hasn't been in communication with a lot of people" -- including her Glee co-stars, an insider said. The source added that her castmates are among those who are respectfully giving her space at this time, as "they know that she needs to be by herself."

The insider said that Michele's "only talking to a few confidantes" during this rough time, as