Earlier in the day Gainer said that mail containing a suspicious "granular substance" was found in the Senate mail.
The letters were addressed to U.S. senators. Each letter was postmarked from Omaha, Neb. on July 25. After the substance was discovered, a memo was sent to staffers.
"Unfortunately, there continues [sic] to be people out there who wish to express their views in this dangerous and alarming fashion," Gainer said in the earlier memo. "As long as we collectively adhere to the procedures, we have the best opportunity to remain safe from those who wish to do us harm."