Several more Senate state offices have received “threatening mail containing a suspicious powdery substance,” though the messages were found to be harmless, the Senate’s top law enforcement officer told CBS News Thursday.

The latest incidents follow a string of similar mailings to congressional offices outside Washington and media organizations that also proved to be harmless, according to authorities.

Still, Sergeant-at-Arms Terrance Gainer said “it is clear that the person sending these letters is organized and committed, and the potential to do harm remains very real.”

On Wednesday, letters were sent to three state and home district offices, including a district office of House Speaker John Boehner, R-Ohio, according to his spokesman.

Letters were also sent to several media organizations, including Comedy Central comedians Jon Steward and Stephen Colbert.

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