The strain of anthrax sent to Senate Majority Leader Tom Daschle was so pure that it's clear the perpetrators know their stuff, according to a government official.

"This isn't a bunch of amateurs," the official, who took part in a Senate briefing on the Daschle letter, said.

Sen. Olympia Snowe, R-Maine, said the strain of anthrax found in the letter was "very refined, very pure."

"Obviously, these are difficult times," said Daschle, D-S.D., as the Senate — and the nation it represents — grappled with the unsettling threat of bioterrorism.

Investigators have found that the strain of anthrax on the letter mailed to Daschle's office was "a very potent form of anthrax that clearly was produced by someone who knew what he or she was doing," the majority leader said.

Three government sources, all speaking on condition of anonymity, said preliminary testing indicated the anthrax had been refined enough so that it could be easily dispersed through the air. One law enforcement official, also speaking on condition of anonymity, said the anthrax was in a purified form that could be used as a weapon. Additional testing was being done late Tuesday.

In an interview with The Associated Press, Tom Ridge, the nation's first director of homeland security, said he suspected the anthrax contamination is linked to the Sept. 11 attacks and suspected terrorist Usama bin Laden's Al Qaeda terror network.

"To me, it's just beyond coincidence," Ridge said. "It's more than coincidence, and we don't have the credible evidence. It's somewhere in between."

Ridge said he gets regular intelligence, law enforcement and military briefings. "As the evidence unwinds, there may end up being a formal tie" between the anthrax cases and bin Laden.

Police cordoned off the southeast corner of the Hart Senate Office Building about a block from the Capitol. It houses the offices of 12 senators including Daschle. Police also checked the building's ventilation system, but at this point anthrax had only been found on the letter. The offices of 12 senators in the Hart office building have been closed for now.

"There will be several hundred people that will be screened today," said Dr. John Eisold, the attending physician at the Capitol.

All of them will receive enough antibiotics to protect them until test results are known. Those people included staff, police, cleaning crews, visitors, and anyone else who had been in the corner of the building that houses Daschle's office. Authorities are offering to test and treat anyone who was near that area.

At a news conference outside the Capitol, police spokesman Dan Nichols repeated numerous times that the only "positive" identification of anthrax spores had come on the mail itself.

FBI Director Robert Mueller said there's a link between the Daschle letter and one sent to NBC anchorman Tom Brokaw. Brokaw's personal assistant is now infected with anthrax.

Daschle's office may not be the only congressional office exposed to anthrax.

Capitol Police officials said they responded to repeated reports of suspicious mail. Sen. Frank Murkowski, R-Alaska, said his aides reported a suspicious letter Monday afternoon and were told by police that their report was the 12th of the day.

In Pawtucket, R.I., eight employees in Rep. Patrick Kennedy's district office were examined for possible anthrax exposure and one worker was tested after she developed a skin rash. Test results weren't expected until Wednesday or Thursday.

Administration officials urged people to be vigilant, but not to overreact. There have only been a limited number of cases in a handful of states, and just one death.

White House spokesman Ari Fleischer has ruled out the possibility that the anthrax came from a government stockpile, saying all stockpiled anthrax has been accounted for.

Mail was suspended again Tuesday to all congressional offices and tours through the Capitol were cancelled, but Senate Minority Leader Trent Lott stressed lawmakers are still getting e-mails and phone calls from constituents and others.

Fox News' Catherine Herridge and the AP contributed to this report.