UC Davis Police Chief Matt Carmichael disclosed the criminal probe on Friday although he did not say exactly what investigators were looking into.
Carmichael said officers from the FBI, the Bureau of Alcohol Tobacco and Firearms and several bomb squads spent 20 hours removing potentially hazardous chemicals from the apartment following Thursday's blast.
He said some were too dangerous to transport and were exploded in an open area on campus.
Carmichael would not describe the chemicals or say what the researcher was doing when the explosion occurred. He said no one is in custody.
Dozens of residents who were evacuated from neighboring apartments were allowed to return home on Friday.