Tests on a plastic evidence bag holding an unopened letter sent to Sen. Patrick Leahy, D-Vt., revealed enough anthrax to kill at least two people — meaning that what's inside the still-sealed envelope itself might be enough to murder many more people at the U.S. Senate.

A federal law enforcement official, who requested anonymity, said Tuesday that a single sample taken from the bag contained 23,000 anthrax spores — more than enough for two deadly doses.

The official said that one sample turned up more of the lethal bacteria than was found in any of the other 600 bags inspected by the FBI before the discovery of the Leahy letter.

Meanwhile, Fox News has learned that traces of anthrax were found in three mailrooms belonging to the federal General Services Administration — one in the Crystal City section of Arlington, Va., one in the main headquarters at 1800 F Street and one in a regional office at 7th and D Southwest.

All three locations get mail from the Brentwood postal facility and have been closed since Oct. 26. They will remain closed while they're decontaminated. GSA officials do not expect any cases of anthrax exposure. None have turned up yet, and they said they believe the exposure period has lapsed.

The low-level, secondary-exposure traces found in the GSA mailrooms are believed to have come from cross-contamination of mail sent from Brentwood.

In addition, congressional sources say that trace amounts of anthrax were found in the offices of Sen. Ted Kennedy, D-Mass., and Sen. Christopher Dodd, D-Conn., both housed in the Russell Senate Office Building.

Officials suspect the anthrax got there through contact with anthrax-bearing letters mailed to Leahy or Senate Majority Leader Tom Daschle, D-S.D. So far, anthrax traces have been found in 13 senators' offices besides Daschle's, whose office is the only one known to have actually opened an anthrax letter.

The FBI announced earlier that it is convinced the Leahy letter was sent by the same person who mailed an anthrax-tainted letter to Daschle.

Investigators are looking into the possibility the Leahy letter was misrouted initially, resulting in anthrax contamination at a State Department mail facility that sickened one worker.

In Atlanta, meanwhile, Tom Skinner, spokesman for the Centers for Disease Control and Prevention, said Monday that the agency planned to test a substance found in a letter that the Chilean government said was tainted with anthrax. The government of Chile said the letter was from an American company in Switzerland to a company in downtown Santiago. It declined to identify either company.

The Leahy letter found Friday will be mined for information based on a plan by the FBI, the Army and outside science experts who want to maximize the evidentiary value of the document, the FBI said Monday.

"FBI and Centers for Disease Control investigators hope that this careful, scientifically agreed upon approach will yield clues that will help identify the source," the bureau said in a statement.

It is more important to proceed with care than with haste, in view of the possibility that the letter could contain "a wealth of other evidence" such as fingerprints, ink and handwriting, a government official, speaking on condition of anonymity, said Monday night.

The Leahy letter, postmarked Oct. 9 in Trenton, N.J., was found by the FBI and hazardous materials personnel from the Environmental Protection Agency in one of 280 barrels of unopened mail sent to Capitol Hill and held since the discovery last month of the letter to Daschle. The Daschle letter also was postmarked Oct. 9 in Trenton.

The outside of the Leahy letter appears virtually identical to the Daschle letter and bears the same fictitious "Greendale School" return address, all-capital block letters and other characteristics.

The matching characteristics of the Leahy and Daschle letters "have combined to convince investigators" that both were "sent by the same person," the FBI said.

U.S. postal inspector Dan Mihalko said the Leahy letter contains a handwritten ZIP code of 20510 that may have been read as 20520 by optical character reader machines at the postal service.

"That's the exact change needed to forward something to the State Department," Mihalko said.

"It raises an interesting possibility that the letter to Leahy could have been misdirected through the State Department mail system initially, which might explain how that system got contaminated," he added.

A 59-year-old employee of the State Department's mail facility in Sterling, Va., was hospitalized Oct. 25 after lab tests confirmed he had inhalation anthrax. He recovered.

On Capitol Hill, the Dirksen and Russell Senate Office buildings reopened Monday after being swept for anthrax contamination after the discovery of the Leahy letter. The Hart Senate Office Building remained closed.

EPA officials have said it will take three to four weeks to decontaminate the offices of 10 senators in the Hart building in which traces of anthrax have been found, a Senate aide speaking on condition of anonymity said. Those cleanups have not yet started.

Two other offices where bacteria were found — Daschle's and the next-door suite of Sen. Russell Feingold, D-Wis. — will be sealed and cleaned with chlorine dioxide gas.

Officials originally hoped Hart, which houses half the Senate's 100 members, would be cleaned and reopened by Nov. 21. With the new cleanup timetable, authorities have set no new target date, but many aides believe the building may not reopen until next year.

The FBI said all congressional mail set aside after discovery of the Daschle letter had been inspected, and the Leahy letter was the only suspicious piece.

Four people have died from anthrax: two Washington postal workers, a hospital employee in New York City and a newspaper photo editor in Florida.

Fox News' Rita Cosby and the Associated Press contributed to this report