Investigators are trying to determine if a mysterious "Fallen Angel" (search) who sent two threatening letters containing ricin (search) last fall is responsible for the deadly poison that turned up in the Senate this week.

The earlier typewritten letters addressed to the White House and Transportation Department warned that more ricin would be used unless new federal trucking regulations were scrapped. The change in 60-year-old rules governing how often truck drivers must rest went into effect Jan. 4.

Three senior federal law enforcement officials, speaking on condition of anonymity Wednesday, said the FBI and Capitol Police Department were investigating the possibility that the same person or persons sent ricin-laced mail to Senate Majority Leader Bill Frist (search), R-Tenn.

Hazardous materials teams from the FBI and Capitol police searched for a letter or parcel that might have carried the ricin powder, which was found Monday in a mail-sorting room in Frist's personal office. The ricin appeared limited to Frist's office in the Dirksen Senate Office Building. No one has been sickened by the poison.

Although three Senate buildings were closed for a second day, Frist announced that they would begin opening on Thursday and the Dirksen building on Monday.

Capitol Police Chief Terrance Gainer said investigators have found "no obvious direct connection" between the Frist incident and the letters signed "Fallen Angel." Those letters were discovered in mail facilities that serve the Greenville-Spartanburg International Airport in South Carolina and the White House. They were found Oct. 15 and Nov. 6, respectively, though the existence of the White House letter was not disclosed by the Bush administration until Tuesday.

The letters, described as nearly identical, claimed that the author owned a tanker truck fleet company and demanded that hours of service rules for drivers remain unchanged, according to the FBI.

The FBI said the South Carolina letter was contained in an envelope with a typewritten warning "Caution RICIN POISON." The letter included claims that the author could make much more ricin and would "start dumping" if the new regulations weren't abolished.

The envelope contained no delivery address and no postmark.

No one has fallen ill as a result of any of the letters. Ricin is a highly toxic substance that is relatively easy to make from castor beans. There is no known antidote but ricin is considered a less effective weapon for causing mass casualties than anthrax, which was mailed to Senate offices in late 2001, because it is more difficult to make airborne and requires inhalation of large quantities to be fatal.

The FBI focused on ricin in its weekly intelligence bulletin to 18,000 state and local law enforcement agencies. The confidential bulletin, obtained by The Associated Press, said no threat of any kind had been received in the Frist case. It concentrated mostly on the dangers of ricin and how police should respond to potential contamination.

The trucking industry has been working with the FBI and Transportation Department inspector general's office on the investigation. The American Trucking Association has sent several bulletins to its members urging them to be on the lookout for people "displaying aggressive behavior" or engaging in suspicious activity.

One association bulletin asked that members "be alert for either a potential disgruntled trucking company, trucking company employee or person purporting to be from the trucking industry" who has made threats in the past against government agencies.

The regulations at the heart of the "Fallen Angel" letters were four years in the making and drew some 53,000 comments when first proposed, trucking association spokesman Mike Russell said. Many truckers and companies were concerned about lost pay and productivity because of stricter rest requirements.

"It was controversial," Russell said.

While the South Carolina letter's existence was made public shortly after it was found, the Bush administration delayed acknowledgment of the White House letter by nearly three months. It was intercepted Nov. 6 by the Secret Service at an offsite mail facility.

Secret Service spokeswoman Ann Roman said that after the letter tested probable for ricin on Nov. 12, the FBI and other agencies were notified. White House spokesman Scott McClellan said Bush administration Homeland Security officials held a Nov. 13 conference call with the FBI, Centers for Disease Control and Prevention, Postal Service and other agencies to discuss what to do.

Ultimately, the ricin in that letter was deemed to be of a low grade and not a threat to public health, so no announcement was made. President Bush was not immediately informed, McClellan said.

"We share information appropriately, if there is a public health risk," McClellan told reporters.

The Al Qaeda terror group has threatened to use ricin, but officials have found no indication that the two "Fallen Angel" letters or the Frist incident are connected to international terrorism.

The FBI has offered a $100,000 reward for information leading to an arrest in the "Fallen Angel" case.