DENVER (Reuters) - Federal prosecutors have charged a Colorado man with threatening gun violence and arson against staff of U.S. Senator Michael Bennet in the days before an unrelated shooting of an Arizona congresswoman.

U.S. Senator Michael Bennet during his swearing-in by Vice President Joe Biden in the Old Senate Chamber, January 5, 2011. Federal prosecutors have charged a Colorado man with threatening gun violence and arson against staff of Bennet. His wife Susan Daggett is seen next to him. REUTERS/Jonathan Ernst

John Troy Davis, 44, made a brief appearance before a federal magistrate in U.S. District Court in Denver on Monday and will remain in custody until his next court date on Thursday to set terms of his bond.

The case comes amid heightened concerns for the safety of lawmakers after Saturday’s attack on Democratic Representative Gabrielle Giffords. Six people died in that shooting in Tucson and Giffords is in critical condition after a bullet passed through her brain.

Handcuffed to a waist-chain, Davis answered “Yes, sir” when asked by the judge if he understood the proceedings. He then asked “Who’s my lawyer?” and was assigned a public defender.

He is charged with one count of assault on a federal employee stemming from a series of threats he is accused of making over the telephone to three staff members at the Denver office of Bennet, a Democrat.

Davis was arrested over the weekend. If convicted, he faces up to 10 years in prison.

According to an FBI affidavit filed with the criminal complaint, Davis was known to Bennet’s staff for frequent calls to his office seeking help with problems he said he was having with his Social Security benefits.

The FBI says he grew increasingly irate in recent calls, telling a staff member last Thursday he was schizophrenic and was “going to come down there and shoot you all.”

He threatened in a separate call to “go down there and set fire to the perimeter” and stated he “may go to terrorism,” the FBI said.

Investigations of such threats are relatively common. Federal prosecutors said they had charged two people with making separate threats against U.S. President Barack Obama last week alone.

The FBI said extra security was provided at Bennet’s home and office in Denver because of the threats.

Bennet was appointed to the Senate in 2009 to fill the seat vacated when Ken Salazar was named U.S. interior secretary. He was elected to a six-year term in November in a race against Republican Ken Buck.