A multi-agency investigation was under way today to determine the cause of an explosion that killed a Los Angeles firefighter, seriously injured another, left a civilian with minor injuries and heavily damaged a credit union building in Westchester.

Fatally injured in the last of three blasts yesterday was firefighter Brent A. Lovrien, 35, a 10-year veteran who died shortly after arriving at Centinela Freeman Regional Medical Center, LAFD spokesman Brian Humphrey said.