A powerful Republican senator is adding his voice to a growing chorus of congressional critics who contend that the FBI may have botched its investigation of the anthrax-laden mailings that killed at least five people in 2001 and have not been solved.

Senator Grassley of Iowa wrote to Attorney General Alberto Gonzales late Monday demanding an update on the status of the probe and expressing frustration at a decision by the Justice Department and the FBI to stop briefing Congress about the investigation.

“How do they expect Congress to do our constitutional responsibility of oversight?” Mr. Grassley asked yesterday in an interview with The New York Sun. “Do they think we’re not supposed to be part of the checks and balances anymore?”

Last month, the FBI refused a request from Rep. Rush Holt, a Democrat of New Jersey, for a briefing on the status of the probe. The agency said no further briefings would be offered to members of Congress because of press accounts that followed earlier briefings and cited congressional sources.

In his letter, Mr. Grassley said he was “shocked” by the explanation because leaks from FBI sources led to news reports fingering a former Army scientist, Steven Hatfill, who was never charged. “For the FBI to withhold information from Congress for fear of leaks seems a bit hypocritical, to say the least,” the senator wrote.

Officials at the FBI and Justice Department said they had received the letter but had no immediate comment. Mr. Grassley said “irritation” with the FBI contributed to his decision to take the matter directly to Mr. Gonzales. “We ought to have an answer as to why this crime hasn’t been solved,” the senator said.

In a letter last month to Mr. Holt, the FBI disputed reports that it operated for years under the mistaken impression that the anthrax used in the mailings was of a refined type that only a few scientists could have produced.

“The FBI and its partners in this investigation have never been under any misconception about the character of the anthrax used in the attacks,” an FBI official, Eleni Kalisch, wrote. She said 17 FBI agents and 10 postal inspectors still were assigned to the nearly five-year-old investigation.

Mr. Grassley said that if Mr. Gonzales does not respond, Congress could consider other approaches, such as subpoenaing the information. “I also think holding up nominations for the Justice Department is a possibility,” the senator said.

Mr. Hatfill is pursuing two libel suits over reports tying him to the anthrax attacks. He is also pressing a Privacy Act case against the federal government over alleged leaks. The New York Times, which is the defendant in one of the libel cases, has acknowledged that two of its confidential sources for columns about Mr. Hatfill were FBI employees. Last week, a federal magistrate ordered the Times to identify those sources and others, but the decision is under appeal.