PRINCETON, Texas -- A 46-year-old single mother was shot to death inside her home after a love-triangle ended with a murder-for-hire, according to police documents.

Kelli Underwood's adult daughter found Underwood inside her Princeton home with a gunshot wound to the head the night of September 20, police said.

It was the first murder in Princeton in at least 20 years, said Police Chief James Waters.

Nearly two months later, with the help of DPS Texas Rangers and the Collin County Sheriff's Office, four people have been arrested and charged with capital murder in the case.

"You could tell the trauma in the daughter’s face, the neighbor’s face," said Waters. "For their sake, I hope it brings them a little bit of closure, I know there’s a lot of emotion."

According to police documents, Ronnie Welborn, 45, and his live-in girlfriend Katie Robinson, 28, paid Samuel Veal, 35, and Delvin Powell, 38, $8,000 to kill Underwood and even provided the gun, a .22 caliber semi-automatic handgun.

Welborn was Underwood's boss and longtime lover, said police. He told police he would see her every Thursday, and sometimes twice a week. According to police reports, Welborn's girlfriend recently had a baby with him and found out that he was cheating. She told police that at first, she had wanted to plant drugs to get Underwood arrested, but when that didn't work, she and Welborn decided to have her killed.

According to police documents, "Robinson felt this was the only way to mend her relationship with her child's father after giving him the ultimatum recently. Robinson said her relationship with Welborn improved since Underwood's murder."

Veal was one of Welborn's former workers and was helping Underwood build a pool in her backyard. He also brought Powell with him the night Underwood was killed, according to police documents.

Welborn put the money in the freezer for Veal to get at his business, according to police documents.

Powell was initially arrested on a separate assault charge. Police found ammo in his girlfriend's trunk that matched the caliber and brand found at the scene of Underwood's murder, as well as a knife with what appeared to be a blood stain, according to police documents. Police said Underwood's autopsy showed she had also been stabbed. Powell later told two other inmates while jailed that he killed a woman, according to police documents.

Each of the four suspects remains in jail on a $1 million bond. The investigation continues, and police are still working to interview them.

"It’s the unimaginable," said neighbor Charles Sewell. "You wouldn’t think it would happen in your neighborhood, but it does and it’s a very sad story."