WASHINGTON (AP) — Investigators are trying to determine if a mysterious “Fallen Angel” who sent two threatening letters containing ricin 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, 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.