NEW ORLEANS (AP) — The top federal prosecutor for New Orleans removed himself on Monday from the case of four conservative activists arrested last week while trying to capture hidden camera footage in a senator’s office.

The Department of Justice said in a news release Monday that Jim Letten, U.S. Attorney for the Eastern District of Louisiana, has recused himself from the case involving the incident in Democrat Sen. Mary Landrieu’s office in New Orleans. Letten’s top lieutenant, assistant U.S. Attorney Jan Mann, is taking over the case.

The news release didn’t say why Letten was removing himself, and a spokeswoman for Letten said she couldn’t comment on the reason.

One of the suspects is the son of Letten’s Shreveport-based counterpart.

Another man who was arrested, James O’Keefe, has said the group wanted to investigate complaints that constituents calling Landrieu’s office couldn’t get through. Landrieu’s office has said O’Keefe’s explanation is feeble, and the case should be thoroughly investigated.

In the Jan. 25 incident, authorities said O’Keefe used his cell phone to try to capture video of two other men who posed as telephone repairmen and asked to see the phones at Landrieu’s office. The fourth is accused of waiting outside in a car with a listening device that could pick up transmissions.

O’Keefe is known nationally for hidden-camera videos targeting the community-organizing group ACORN.

Also arrested were Robert Flanagan of New Orleans, Joseph Basel of Minnesota and Stan Dai of the Washington, D.C., area, all 24. They face a charge of entering federal property under false pretenses for the purpose of committing a felony, which carries up to 10 years in prison. They are free on $10,000 bail and are due back in court Feb. 12.

Flanagan is the son of the Shreveport-based acting U.S. attorney for Louisiana’s Western District.