Congressional offices have once again become the target of “threatening mail,” with one House district office and two Senate state offices receiving letters containing “a suspicious powdery substance.”

The House and Senate Sergeants-at-Arms circulated internal memos via email today informing Members and staff of the incidents, which occurred on Tuesday.

While those letters were tested and found harmless, the Senate Sergeant-at-Arms this afternoon announce the arrival of a second suspicious letter to a Senate state office.

The office of the Senate Sergeant-at-Arms said the author of the letters indicated that additional letters would be arriving at more Senate offices, and that some might contain dangerous materials.

The House Sergeant-at-Arms’ office, in its memo, also confirmed that additional letters could be in the mail stream en route to House district offices.

“These incidents are reminders that we must remain vigilant in handling mail, recognizing suspicious items and knowing what immediate actions to employ if faced with suspicious mail in the office,” read both memos, which were obtained by Roll Call.

Both emails also warn that further threatening letters will likely bear a return address of “The MIB, L.L.C.,” located in Portland, Ore.

“If any mail is received from this return address, it should remain unopened and the local authorities contacted immediately, followed by notification to the United States Capitol Police Threat Assessment Sections,” the memos caution.

Officials would not disclose the names of the lawmakers whose offices received the letters.

A press officer with the FBI Washington field office told Roll Call that the FBI’s national headquarters will be working with Capitol Police to investigate.

These incidents follow Friday’s joint arrest by the FBI and Capitol Police of Amine el Khalifi, a 29-year-old Moroccan immigrant with an expired visa who was allegedly attempting to blow himself up in the Capitol. An FBI affidavit states that el Khalifi thought his collaborators were with al-Qaida, but they were actually undercover agents supplying him with nonfunctioning weapons.

El Khalifi made his second official appearance in the U.S. District Court for the Eastern District of Virginia today. Under representation by a public defender, he waived his rights to preliminary and detention hearings. U.S. Magistrate Judge John Anderson ordered el Khalifi held without bail pending trial on grounds that “there is a serious risk the defendant will not appear [and] will endanger the safety of another person in the community.”