WASHINGTON, 26 May — Tensions are rising among mid-level US government leaders over the investigation of last September’s hijacked airliner attacks on the United States. Anger over the matter has created a revolt among Democrats on Capitol Hill and among mid-level agents in the Federal Bureau of Investigation (FBI), prompting FBI field agents to make dark jokes that “Osama Bin Laden has a ‘mole’” at the bureau’s Washington headquarters.

Last August, agents in the FBI’s Minneapolis field office unsuccessfully sought a warrant to search flight school student Zacarias Moussaoui’s computer after learning that similar concerns were raised three weeks earlier about other suspected terrorists in Arizona by their FBI colleagues in Phoenix.

Despite information that suspected Al-Qaeda terrorists were involved in flight training in two states, the warrant request — coming a month before the Sept. 11 attacks — was rejected by FBI officials in Washington for “a lack of probable cause.”

That decision, according to government sources, so infuriated the Minneapolis agents that they joked among themselves that Osama Bin Laden, whose Al-Qaeda network has been named in the Sept. 11 attacks, “must have successfully planted a mole inside the FBI.”

The Minneapolis agents arrested Moussaoui in August after questions surfaced about his flight training. The 33-year-old French-Moroccan has since been charged in the attacks on the World Trade Center and the Pentagon that killed at least 3,000 people. Authorities named him as “the 20th hijacker” — although he was already in custody at the time of the attacks.

The Phoenix memo, by FBI Agent Kenneth Williams, alerted bureau executives in Washington in July that Bin Laden followers were training at an Arizona flight school. He said eight suspected terrorists were training, but senior FBI officials did not follow up on the information.

The decision to reject the Moussaoui warrant was criticized in a 13-page letter to FBI Director Robert S. Mueller III from Minneapolis Agent Coleen Rowley, who said senior FBI officials created a “roadblock” to derail the probe. She said the agents were so frustrated by the lack of response by senior officials that they sought to bypass the chain of command and directly notify the US Central Intelligence Agency (CIA) — but were reprimanded.

Rowley said that although they were “closest to the action and in the best position to gauge the situation locally,” the agents were not allowed to proceed despite the corroborating Phoenix alert and information they had from intelligence officials overseas that Moussaoui was tied to terrorism suspects.

“There was a great deal of frustration expressed on the part of the Minneapolis office toward what they viewed as a less-than-aggressive attitude from headquarters,” Rowley said. “The bottom line is that headquarters was the problem.”

Sen. Charles E. Grassley (R-Iowa), a senior member of the Senate Judiciary Committee, said he was “alarmed” by the Rowley letter. He said if the decisions by senior FBI officials in the Moussaoui probe were an example of how the FBI was investigating terrorism, “We’re in grave danger.”

But while Sen. Grassley clearly supports a stepped up investigation of how the FBI, the CIA and other US government agencies handled events surrounding the September attacks, the Senate Democrat leading an investigation of 9-11-related intelligence failures defended the congressional investigation on Friday, just minutes after Senate Majority Leader Tom Daschle called for an independent inquiry.

“We are going to carry out our responsibility to do the overview of the intelligence community,” said Sen. Bob Graham (D-Florida), chairman of the Senate Select Committee on Intelligence. “And what people decide to do otherwise is their prerogative.”

Graham said the House and Senate intelligence panels are conducting an “aggressive” joint investigation of the FBI and the CIA, and will begin hearings June 4.

But Daschle (D-South Dakota) cited Rowley’s angry letter to the FBI leadership as “yet another illustration of why a commission is necessary.”

“I think there’s a growing realization that a commission is required,” Daschle said.