A few weeks after suspicious packages began to trickle into Senate offices on and off Capitol Hill, the perpetrator is still at large, according to the chamber’s chief law enforcement officer.

In an internal memo emailed this evening to the Senate community and obtained by Roll Call, Senate Sergeant-at-Arms Terrance Gainer reported that the unknown sender based in the Pacific Northwest could still be targeting lawmakers in both their state offices and Washington, D.C., headquarters.

Letters containing powdery substances have surfaced in dozens of Senate offices. All substances were found to be harmless, and a joint investigation by the FBI, U.S. Postal Inspection Service, Capitol Police and local law enforcement agencies is ongoing.

But general uncertainty as to what might come next is reason to be on guard, Gainer wrote.

“The fact remains that we do not know who the offender is. Just as important, we do not know for certain if more letters are on the way,” he wrote. “The letter writer has warned that there could be additional mailings containing potentially dangerous substances.”

His email primarily served as an update on the situation, but Gainer also reminded lawmakers and staff how to handle mail that they think might contain hazardous materials and to be wary of letters postmarked from Portland, Ore.

“This recent episode serves as a real-world reminder of the importance of mail safety to the Senate community every day of the year,” Gainer wrote. “Ever since the deadly Anthrax attacks of 2001 and the Ricin incident [two and a half] years later, the Senate Post Office has implemented and refined a strong mail-safety program.

“However, that program is only as effective as the people who make it work,” he added.

Senators who have received suspicious mailings include Dan Coats (R-Ind.), Patrick Leahy (D-Vt.), Joe Lieberman (I-Conn.), Patty Murray (D-Wash.), Pat Roberts (R-Kan.), Pat Toomey (R-Pa.), Johnny Isakson (R-Ga.), Scott Brown (R-Mass.), Carl Levin (D-Mich.) and Barbara Mikulski (D-Md.).

So far, only one House Member has received a powdery substance in his district office: Speaker John Boehner (R-Ohio).