The bus, transporting 44 high school students to Humboldt State University for a familiarization visit, collided with a FedEx truck, killing 10 people—five students, the three adults accompanying them, and both drivers—and injuring dozens.
The investigation will look into the use of seatbelts, emergency exits and fire safety rules on buses.
The National Transportation Safety Board (NTSB) will provide an initial report within 30 days but the investigation could take up to 12 months. The NTSB has long advocated increased safety for bus passengers but other federal agencies have been slow to respond.
"The worst thing for the NTSB is to show up, know that we've issued recommendations from a previous accident where lives have been lost ... and find out (that) if those recommendations had been closed and enacted, lives could have been saved," NTSB member Mark Rosekind told AP on Friday.