Just when the anthrax scare seemed to have faded, two Senate office buildings were closed over the weekend and a letter suspected of containing the toxic substance was sent off to a lab for testing.

The letter was addressed to Sen. Patrick Leahy, jarring Congress and the nation at a time when the anthrax-by-mail threat was thought to have subsided.

"It's kind of chilling when you see your name on something like this," said Leahy, D-Vt.

Following the discovery, the FBI renewed its appeal for help in tracking down the culprit responsible for sending anthrax through the mail.

"This is a cold-blooded murderer," said Van Harp, assistant director in charge of the FBI's Washington Field Office. The bureau is offering a $1.2 million reward.

Four people — two postal workers in Washington, D.C., a New York City hospital worker and a Florida photo editor — have died from exposure to the bacteria.

The unopened envelope sent to Leahy resembled one mailed last month to Senate Majority Leader Tom Daschle, D-S.D. The Leahy letter was discovered in the 280 barrels of congressional mail quarantined after a Daschle employee opened a powder-filled envelope Oct. 15.

The latest suspicious piece of mail was sealed and sent to the Army installation at Fort Detrick, Md., for tests. It was not known when results would be available.

"At this juncture we do not know that it's anthrax," Harp said at a news conference with Leahy.

The senator's chief of staff, Luke Albee, said it wasn't clear whether the letter ever reached Leahy's office or whether any of the senator's staff had touched it.

The discovery led to a renewed shutdown of the Russell and Dirksen Senate office buildings for tests to determine if anthrax spores were present.

Everything on the envelope except the name and address was similar to the Daschle letter: the block printing with a slight slant to the right; the Oct. 9 postmark from Trenton, N.J.; the same, nonexistent school listed as the return address.

All congressional mail set aside after discovery of the Daschle letter has been inspected, and the Leahy letter was the only suspicious piece, Harp said.

The Daschle letter led to the partial shutdown of the Capitol, the closing of congressional office buildings and the testing of hundreds of employees.

No congressional staff member or lawmaker has contracted anthrax, and business for the most part has returned to normal on Capitol Hill -- despite the introduction of National Guard troops this weekend to help overburdened Capitol Police.

Leahy said the letter was a scary reminder of the danger faced by people on Capitol Hill. The senator's staff was told Saturday there was no immediate reason to begin taking antibiotics.

His spokesman, David Carle, said that the office decided on its own Oct. 12 to set aside all unopened mail after an anthrax-contaminated letter was sent to NBC anchor Tom Brokaw.

Lt. Dan Nichols of the Capitol Police has warned repeatedly that another letter with anthrax could be found in the congressional mail that was set aside.

Officials believe there is at least one other undiscovered anthrax letter hiding in stacks of State Department mail.

The department is beginning to sort through three weeks' worth of mail that have piled up since a department mail handler became ill sick with inhalation anthrax and the mail was stopped.

The Associated Press contributed to this report.