FBI agents and EPA Region 9 staff have been going door to door in the city of Martinez, asking residents for details about the release of metal-laden dust from the Martinez Refining Co. over the Thanksgiving holiday last year.

An FBI spokespeson confirmed Friday that the agents were canvassing residents as part of a joint investigation, but referred all other inquiries to the EPA.

“EPA is communicating with local, state, and federal agencies and does not comment on any ongoing investigations,” said Michael Brogan, a spokesperson for EPA Region 9.

Martinez Refining, located on an 880-acre industrial complex on the northern edge of the city, emitted as much as 24 tons of so-called spent catalyst, a mix of chemicals used to break down crude oil into finished petroleum products like gasoline, according to the local air district.

The fallout left cars, homes and at least one school blanketed in a white powdery substance. Tests determined that the residue contained metals such as aluminum, barium, chromium, nickel, vanadium and zinc.

Martinez Refining did not immediately inform county officials about the chemical release as required by law, according to Contra Costa County Health Services. The local air district and county officials learned after receiving complaints from residents.

The health department later advised community members not to eat foods grown in the soil if their homes were dusted by the spent catalyst.

The entry of federal investigators has stunned Martinez residents who are still awaiting the county-ordered soil testing and investigations by other local agencies.“We kind of expected quiet investigations in the background. But to have the FBI come out, that was never on our radar at all,” said one Martinez resident who spoke with federal investigators and asked to remain anonymous out of fear of retaliation.

The county health department has referred two violations to the Contra Costa County district attorney’s office — one for failure to notify the proper authorities of a hazardous material release and one for illicit discharges into the county stormwater system. Both referrals remain under review.