A senior Republican senator said Wednesday that Lanny Breuer, head of the Justice Department’s criminal division, should step down over a troubled operation that targeted gun smuggling in Mexico.

Attorney General Eric H. Holder Jr. “needs to ask for Mr. Breuer’s resignation and remove him from office if he refuses,” Sen. Charles E. Grassley (R-Iowa) said in prepared remarks.

The “Fast and Furious” program was meant to track guns as they made their way south of the border to senior members of Mexican drug cartels after being bought by straw buyers. However, ATF agents rarely pursued the weapons after they were bought.

Grassley said Breuer hurt his own credibility by initially denying details of the program.

In a statement, Justice Department spokeswoman Tracy Schmaler said Breuer had “acknowledged his mistake” in not alerting the department’s leadership about the tactics used.

At a hearing in the Senate last month, Holder expressed regret over both the operation and providing information to Congress about it that later turned out to not be true. Holder at the same hearing maintained his support for Breuer. “I don’t expect to hear a resignation offer from Mr. Breuer,” he said at the hearing.