DUCHESNE — A former Salt Lake County prosecutor, who is already facing a number of felony charges, stands accused of assaulting his girlfriend during an argument at the state courthouse in Duchesne.

On Oct. 17, a Murray woman told Duchesne County sheriff's deputies she was at the 8th District courthouse to address a warrant that had been issued for her arrest in a drug possession case.

She said she was confronted by her boyfriend, Matthew George Nielsen, who was also at the courthouse to represent a client in an unrelated case. The pair went into a meeting room and began to argue, according to court records.

"This argument became physical, and (Nielsen) slapped her on the face and pushed her to the back of the room and held her by the neck," charging documents state.

Deputies were told Nielsen threatened to kill the woman and her child, then knocked her to the floor.

"She hit her head, making a loud noise" that was heard by a court security officer who entered the room and separated the couple, but took no further action, court records state.

Nielsen's defense attorney, Ed Brass, questioned the woman's account of what happened when contacted Wednesday. He said his client is embroiled in a "tumultuous relationship" with a woman who continues to "go to the authorities and come up with a story that may or may not be true."

Nielsen told deputies he did not assault the woman when he was arrested several hours after the courthouse incident. A search of his pickup truck following a traffic stop turned up several loose prescription pills, investigators said.

Duchesne County prosecutors charged Nielsen on Tuesday with witness tampering and possession of a controlled substance, both third-degree felonies. He was also charged with misdemeanor counts of assault, reckless driving, drug possession, and driving with a measurable amount of a controlled substance in his system.

It's not the first time, though, that Nielsen has faced legal trouble.

In 2008, he left his job with the Salt Lake County District Attorney's Office and was charged that same year with five counts of forging a prescription for a controlled substance.

Nielsen entered a no-contest plea to one of the charges and entered the Tooele County Drug Court program, which he successfully completed, court records show.

In 2012, Nielsen was charged in 3rd District Court with four counts of falsely obtaining or dispensing a controlled substance. That case is set for trial in December.

Nielsen was also charged in September 2012 with witness tampering, child abuse, domestic violence in the presence of a child and making threats of domestic violence. Those charges came after police were called to Nielsen's home in Cottonwood Heights on a report that he was beating his girlfriend.

Nielsen's teenage daughter and son told police they tried to stop the alleged assault, but he chased after them, grabbed his daughter "and began punching her in the chest and stomach" before telling her, "If you call the police, I'll kill you," the charges state.

Nielsen also threw his son against the wall and hit him when he tried to help his sister, according to prosecutors in the case.

The children eventually called their mother, Nielsen's ex-wife, to pick them up. As they were leaving, Nielsen threw a bag at his daughter, hitting her in the back, according to the warrant. He also threatened to kill his ex-wife when she arrived, the charges state.

That incident came just a month after Nielsen was arrested in a separate case involving the same girlfriend. Nielsen punched the woman and knocked her out, then dragged her into the garage where he poured beer on her, according to court documents.

When the woman tried to make a run from Nielsen's house, he sprayed her with mace, the charges state. The woman continued running to a neighbor's house for help.

A misdemeanor battery charged filed in that case was dismissed in April, when Nielsen's girlfriend told prosecutors she would not testify against him, court records show.

Nielsen's first court appearance in the Duchesne County case is set for Nov. 7.

With all the charges against his client, Brass knows people will assume Nielsen is someone who is spiraling out of control.

"That's the way it would look on paper," Brass said, "but it should be clear that Mr. Nielsen asserts his innocence with respect to all these charges."

Email: gliesik@deseretnews.com

Twitter: GeoffLiesik