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Nobody was ever put on trial for those mailings, though prosecutors identified Bruce E. Ivins, a scientist who had worked at a biodefense lab at Fort Detrick in Frederick, Md., as the culprit. As prosecutors were preparing to charge him, Ivins committed suicide by taking an overdose of acetaminophen.

In the wake of that incident, all mail to the Capitol is now received and tested at the off-site facility.

In his letter to Senate employees Tuesday, Mr. Gainor warned those who work in offices in the states to use Postal Sentry, a device that is supposed to decontaminate mail.

“The Postal Sentry is really the only defense available in state offices for this type of threat,” Mr. Gainor said.

⦁ Sean Lengell and Seth McLaughlin contributed to this article, which is based in part on wire service reports.