WASHINGTON, 19 November — Authorities are continuing to conduct tests on the most recently-discovered anthrax-laced letter, in the hope that the correspondence will yield vital clues in the US government’s stymied bioterrorism probe.

Officials yesterday placed the unopened letter — mailed to US Sen. Patrick Leahy — in an airtight bag and delivered it to a US Army medical research lab in Fort Detrick, Maryland outside the US capital, where scientists tested it for anthrax spores.

The letter also will be scoured for forensic evidence, such as fingerprints, hairs or fibers. FBI officials said initial tests suggested that the correspondence contained the deadly microbes, but said final results would not be available until later this week.

US authorities on Saturday urged anyone with information about string of germ attacks to contact police immediately. "Come forward if you know anything; come forward before any more innocent people die," said Leahy.

The FBI is working "aggressively and furiously around the clock" to find the "cold-blooded murderer" behind the anthrax attack, says a top FBI official.

"There are four individuals dead as a result of this, none of them the intended targets… We need the public’s help," Van Harp, assistant director of the FBI’s Washington Metropolitan field office, said during a briefing on Capitol Hill.

"I’m hoping the one bright light might be that the letter would give us further evidence about who is doing this, Sen. Leahy, Chairman of the Judiciary Committee, said yesterday. The letters sent to Sens. Tom Daschle, Senate Majority Leader, and Leahy had similar handwriting — block lettering, slanted to the right, both were postmarked Oct. 9 from Trenton, New Jersey and both had the same return address — a nonexistent elementary school in Trenton, Harp said.

The FBI official told reporters authorities are still not certain the Leahy letter contains anthrax. Asked why he was targeted, Sen. Leahy said he had no idea.

"My name could just as well have been picked out randomly. I think you’re dealing with someone who wants to strike out… I have no idea why my name was on there. It could have been any one of the other 99 senators whose name was on there."

Harp said that his agency believes that there is "someone out in the country somewhere that may have information ... may be aware of what this person is doing, may have a little piece of information that would help complete the puzzle for us."

"This is a cold-blooded murderer," Harp said, pointing out there was a $1.25 million reward for information leading to the arrest of the person behind the anthrax-by-mail attacks that killed four people since early October.

Officials said the letter bolsters their theory that the anthrax attacks are homegrown and unrelated to the terror attacks two months ago on the Pentagon and World Trade Center. "I think this tends to give a little more credence to the fact that this was domestic as opposed to related to Sept. 11," said Dan Mihalko, spokesman for the US Postal Inspection Service.

With little information to go on, FBI agents last week portrayed the person behind the anthrax attacks as a US-based loner with a scientific background.

However, Milton Leitenberg, a biological weapons specialist at the University of Maryland’s Center for International and Security Studies, noted that the choice of the understated and not-well-known Leahy as a target may be a clever bit of subterfuge.

"It still could be, in terms of Sept. 11, someone who is brilliantly pushing us off track by sending it to Leahy," Leitenberg said. The latest letter was found among some 250 barrels of unopened mail that had been sequestered when the congressional postal service shut down in mid-October.