Investigators continued efforts Feb. 4 to identify the letter or parcel that brought ricin to a Senate mailroom and to determine whether the incident is linked to the deadly poison found last fall in letters at mail facilities serving the White House and a South Carolina airport.

A letter with ricin found in October at a postal facility serving the Greenville-Spartanburg International Airport was signed “Fallen Angel.” Another letter with ricin signed the same way was found in November at a facility that processes mail for the White House, according to a federal law-enforcement official who spoke to The Associated Press Feb. 3.

Both letters complained about new hours-of-service regulations for truckers that became effective Jan. 4.

Meanwhile, hazmat teams from the FBI and U.S. Capitol Police could not find a specific letter or parcel that could be connected to the ricin in Senate Majority Leader Bill Frist's office, according to the FBI.

An initial check found no extortion, threat or complaint accompanying the ricin found in Frist's office, The AP reported. There also were no indications of involvement by foreign terrorists such as al Qaida, which the FBI has said was interested in using ricin in an attack.

The powdery white substance was found on a machine that opens mail in Frist's office, authorities said. The area was quarantined and stacks of mail were being checked.

The package found in a South Carolina mail facility had a letter claiming the author, claiming to be a fleet owner of a tanker company, could make more ricin and a threat to “start dumping” large quantities if his demands to stop the new trucking regulations were not met. Federal authorities offered a $100,000 reward in that case, but no arrests have been made.

The White House letter, intercepted in November, contained nearly identical language but such weak amounts of ricin that it was not deemed a major health threat, said another law-enforcement official. That letter's existence was not publicly disclosed before Feb. 3.

The latest discovery comes as the FBI continues its 28-month-old investigation into the fall 2001 mailings of anthrax-laced letters to Senate and news media offices. Five people died and 17 were injured in that attack.