Unopened mail is being collected throughout the Capitol complex by investigators searching for letters that could be contaminated with the deadly poison ricin (search) — a terrorism-era specter that Congress has learned to dread.

Genetic testing by the Centers for Disease Control and Prevention (search) provided further confirmation Wednesday that the white powder found in an office of Senate Majority Leader Bill Frist (search) on Monday was indeed ricin.

In an interview, U.S. Capitol police chief Terrance Gainer said there have been no illnesses reported, no new findings of ricin anywhere else in the Capitol complex and no clues about how the poison got to Frist's mailroom.

Three Senate office buildings remained closed Wednesday after the powder was found, a little more than two years since Congress grappled with the potentially lethal bacteria, anthrax (search).

There couldn't be business as usual, but there was business nonetheless. Senators walked across the Capitol to join their House counterparts in a joint session to hear an address from visiting Spanish Prime Minister Jose Maria Aznar.

A few people briefly re-entered the Dirksen building Wednesday — including the very fourth-floor corridor where the ricin was discovered. An aide to Sen. James Jeffords, I-Vt., whose office is adjacent to Frist's on the building's fourth floor, went inside for less than 10 minutes to remove things. The aide, Laurie Schultz Heim, said she noticed nothing unusual other that two machines in the hallway that seemed to be filtering the air.

"It's completely normal," she said.

Senate aides said at least one office building might be reopened on Thursday, though it would not be Dirksen, where the ricin was found.

Outside the Capitol, police stood guard on street corners around the office buildings and a group of Marines unloaded equipment at an adjacent parking lot. "The Marines are helping us with our efforts in clearing and isolating in the cleanup related to ricin," said Capitol Hill Police spokesman Sgt. Contricia Ford.

About 100 soldiers were deployed from the U.S. Marine Corps' Chemical Biological Incident Response Force, said the unit's spokesman, 1st Lt. Paul C. Cabellon. The force conducts reconnaissance and decontamination at sites where chemical, biological, radiological or nuclear weapons may have been used.

Meantime, law enforcement authorities intensified efforts to identify the letter or parcel that brought ricin to the Senate mailroom while also seeking to determine whether that incident is linked to poison found last fall in letters at mail facilities serving the White House and a South Carolina airport.

Gainer said he was aware of "no obvious direct connection" between the ricin found in Dirksen and in a letter addressed to the White House that was intercepted in November. He also said the Capitol police had known about the White House ricin letter "for weeks and weeks and weeks."

Asked if his force had taken any precautionary steps as a result of the information, he said, "Everything that happens, we re-examine and fine-tune what we do. But everybody knows the limitations of our ability to detect ricin in an envelope."

Hazardous materials teams from the FBI and Capitol Police Department conducted a preliminary search Tuesday night but did not find a letter or parcel that could be connected to the ricin in an office of Senate Majority Leader Bill Frist, FBI spokeswoman Debra Weierman said. The teams were conducting a more methodical search Wednesday.

President Bush was not told about the ricin letter sent to the White House — and there was no public disclosure of the incident — because "it did not pose a public health risk," spokesman Scott McClellan said. "We share information appropriately, if there is a public health risk."

He also defended the handling of the information, despite some reports that the Secret Service delayed sharing it with other federal officials. "We expect people to be notified appropriately and we understand that they were," McClellan said.

Even while some test results were still pending, House Speaker Dennis Hastert said, "It's serious enough that they're picking up everybody's mail."

But dozens of Senate workers were being monitored and health officials urged Senate staff to watch for swiftly developing fever, coughs or fluid in the lungs over the next two or three days. When inhaled in sufficient quantities or injected, ricin can be fatal — and there is no known vaccine or cure.

The postal facility that processes Congress' mail also was shut. In 2001, two postal workers in Washington were among five people who died from anthrax exposure.

The investigation into that letter continues, and there have been no arrests, the official said. Authorities determined the letter posed no threat to health because of the ricin's low potency and granular form.

The letter was similar to another letter containing ricin that showed up in October at a postal facility serving Greenville-Spartanburg International Airport, according to a senior law enforcement official, speaking on condition of anonymity. The author of both letters complained about new regulations requiring certain amounts of rest for truck drivers, the official said.

There was widespread agreement among lawmakers that Congress responded more effectively this week than to the anthrax-laced letters that were sent to two senators in 2001.

But even Frist acknowledged that things were "not perfect." Chief among senators' complaints were that authorities were too slow to alert them.