Pamela Hupp has been charged with first-degree murder after fatally shooting someone inside her O'Fallon, Mo. home.

Hupp shot and killed 33-year-old Louis Gumpenberger of St. Charles, who she claimed was a burglar, on August 16.

During a press conference Tuesday, Prosecuting Attorney Tim Lohmar described what investigators believe happened last week.

When officers arrived to the scene, they found Gumpenberger dead with a handwritten note and $900 cash in his pocket. The note had instructions to kidnap Hupp, "get Russ's money," and kill Hupp.

According to Hupp, a car pulled up to her driveway and Gumpenberger exited the passenger seat before climbing into her car. She says he held a knife to her throat and demanded she take him to the bank to get "Russ's money." Hupp claims she was able to knock the knife out of his hand then run into her home. She says Gumpenberger followed her into the house, threatening her life. That's when she says she called 911 and shot him.

But investigators have a different story.

They believe Hupp was trying to "take the heat" off of herself from the Russ Faria trial.

Police said information taken from Hupp's cell phone placed her in front of Gumpenberger's apartment complex before the 911 call came in. Witnesses at the apartment complex say Gumpenberger had been outside around the same time.

Police believe Hupp likely posed as a Dateline NBC producer and convinced Gumpenberger to come over and reenact 911 calls. They say the 33-year-old man may have fallen for it all because of mental challenges stemming from a 2005 accident.

Lohmar said Gumpenberger had previously suffered a traumatic brain injury and had slurred speech, did not drive, had limited motor function and had no known income. He went on to say Hupp was targeting an innocent victim who could be easily influenced by money.

"She was very calculated and looking for somebody that fit a particular profile. This victim unfortunately fit that profile. Somebody who may not be sophisticated, somebody who might be easily persuaded by a decent amount of cash," said Lohmar. "I don't think she anticipated that it would turn out that he would have the physical and mental disabilities that he did have, and I think that's part of her undoing, to be honest with you."

Before she was formally booked at the police station, officers Hupp attempted to stab herself in the throat and wrists with a ballpoint pen she had snuck into the bathroom. She was taken to the hospital in stable condition.

She has since been released from the hospital and is being held in the St. Charles County Jail on a $2 million cash-only bond.

Last year, Hupp was a key witness at the retrial of Russ Faria, even though she was never called to the stand.

Russ Faria's wife Betsy was found murdered on December 27, 2011 in the couple's Troy, Mo. home. She had been stabbed 55 times.

Hupp, who was reportedly the last person to see Betsy alive, collected $150,000 in life insurance after her death. She had become Betsy's beneficiary several days before the murder.

Though Hupp did not testify in the 2015 retrial, a detective testified Hupp told police that she and Betsy had been lovers, according to NBC News.

Although defense lawyers contended Hupp was an alternate suspect in the Betsy Faria murder case, she was never officially named as a suspect in the case.

Watch the full press conference here.