Bonta said his investigators will partner with federal authorities, whose probe is being led by the U.S. Coast Guard and local officials including Orange County District Attorney Todd Spitzer.
"I'll be taking a leadership role, working with our partners ... we're committed to a collaborative, cooperative approach," Bonta said.
The attorney general said there's an important role for state prosecutors, who could pursue civil or criminal charges, or both.
The oil spill was reported to federal authorities on the morning of Oct. 2. By that time, as much as 131,000 gallons had seeped from a pipeline that runs from an offshore platform to the Port of Long Beach. The pipeline is operated by a subsidiary of Amplify Energy Corp., based in Texas.
"When we get involved and when we use our tools, we hope that we will continue to do what we've always done, which is be accurate, be complete, be thorough, be objective and, most importantly, achieve accountability where necessary and realize justice," Bonta said in announcing his office's investigation.