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A bipartisan compromise that would have expanded federal background checks for firearms purchases has been rejected by the Senate.

The defeat of the measure by a 54-46 vote — six votes shy of the number needed to clear the Senate — marks a major setback for gun-control advocates, many of whom had hoped that Congress would act to curb gun violence in the wake of December's Newtown elementary school massacre, where 20 students and six adults were killed.

Update at 8:44 p.m. ET. Authorities Make An Arrest:

Authorities have made an arrest in connection to the suspicious envelopes sent to a senator and President Obama.

The FBI identified the suspect as Paul Kevin Curtis.

The The Clarion Ledger reports:

Quoting "congressional and law enforcement sources," CNN is reporting that an envelope sent to a senator's office has tested positive for the poison ricin.

"After the envelope tested positive in a first routine test, it was retested two more times, each time coming up positive, the law enforcement source said," CNN reports. "The package was then sent to a Maryland lab for further testing."