The tanker crashed in a remote area of a forest about 30 miles northeast of the city of Chico.
The NTSB took over the investigation yesterday afternoon.
Ellen Engleman-Connors, chairwoman of the agency, said Thursday night the cause of the crash couldn't readily be determined.
"It's too soon to say and we would never speculate. We really focus on facts, science and data and not guess or supposition."
Last May, the federal government terminated contracts for 33 air tankers, citing safety concerns after two planes broke up in midair in 2002.
The plane that crashed Wednesday evening was one of ten approved this week to rejoin the federal firefighting fleet.
(Photo courtesy: NTSB)