A suburban St. Louis woman who claimed she shot an intruder who followed her inside her home is now facing murder charges, with a prosecutor saying Tuesday that she set up an innocent, mentally-impaired man to kill him.

St. Charles County prosecutor Tim Lohmar said Pamela Hupp, 57, of O'Fallon, Missouri, "hatched a plot to find an innocent victim and to murder this innocent victim" as part of an effort to frame a man involved in a previous murder case in which Hupp was a key witness.

Hupp is charged with first-degree murder and armed criminal action in the Aug. 16 death of Louis Gumpenberger of St. Charles, a 33-year-old man left physically and mentally impaired from a 2005 car wreck.

Hupp was arrested at her home Tuesday, and while in custody at the O'Fallon police station stabbed herself multiple times with a ball point pen she had managed to hide, O'Fallon Police Chief Roy Joachimstaler said. She was hospitalized in stable condition. Bond was set at $2 million.

A message left with Hupp's attorney was not immediately returned.

Hupp testified in the 2013 murder trial of Russell Faria in nearby Lincoln County, Missouri. Faria was convicted of fatally stabbing his wife, but the conviction was reversed and Faria was acquitted in 2015. The case was upended partly on claims that Faria should have been able to argue in the first trial that Hupp had a motive to kill Betsy Faria after becoming beneficiary of a $150,000 life insurance policy shortly before Betsy Faria was killed.

Hupp has said she had nothing to do with Betsy Faria's death, but Joachimstaler said police believe Hupp set up the killing of Gumpenberger "to take heat off her because of that previous case."

It wasn't immediately clear if the new charges against Hupp would lead to a new investigation of Betsy Faria's death. A message left with the Lincoln County sheriff's office was not returned.

Hupp told police that soon after pulling into her driveway on Aug. 16, Gumpenberger, who she said was a stranger, jumped out of a car and demanded at knifepoint that she take him to a bank "to get Russ's money," Lohmar said. "Russ" was an apparent reference to Russell Faria and the insurance money Hupp claimed after Betsy Faria's death.

Lohmar said Hupp claimed she knocked the knife out of Gumpenberger's hand and ran inside. He followed her, she said. She went into a bedroom and got a gun, fatally shooting Gumpenberger.

But Lohmar said that six days before the killing, a woman reported that a white female in an SUV approached her claiming to be a producer for the TV show "Dateline" and tried to recruit her to record a scripted sound bite about 911 calls, promising to pay her $1,000. The woman initially agreed but backed out when the woman couldn't produce any credentials. Surveillance footage from a camera on the woman's home captured the SUV's license plates, which matched Hupp's, Lohmar said.

"Our theory is she was vetting a potential victim," Lohmar said.

Meanwhile, GPS from Hupp's cellphone showed that she was at Gumpenberger's apartment, 13 miles from her home, less than an hour before their confrontation on her driveway, Lohmar said. She was there for about four minutes, but authorities don't know what transpired during that time, Lohmar said.

"She was very calculated looking for someone who fit a particular profile," Lohmar said. "This victim fit that profile, someone not very sophisticated, someone easily swayed by a large amount of cash."

Gumpenberger had no cellphone or ID at the time of his death. Police found $900 in plastic bags in his pocket, along with a note that appeared to be instructions to kidnap Hupp and collect "Russ's" money, Lohmar said. But authorities believe the money and note were planted.