ST. GEORGE, Utah — The transgender live-in boyfriend of Charlie Kirk’s alleged assassin appears to have fled their cozy Utah townhouse, leaving mail piling up outside — as he faces mounting criticism for not turning the accused killer in after he confessed to the hateful slaying in a string of text messages.

Lance Twiggs, 22, who has reportedly been cooperating with authorities since his partner, Tyler Robinson, also 22, allegedly shot the Turning Point USA co-founder dead during a speaking event on Sept. 10, has disappeared, neighbors in St. George, Utah, told The Post Tuesday.

Mail was spotted piling up outside the $1,800-per-month townhouse the couple shared, as backlash from the public jeopardized his safety — and cops are still a presence in the quaint community even two weeks after Robinson was cuffed.

Jesse Riley, who lives just a few houses down from the couple, said he hasn’t seen any recent signs of life in the apartment.

“It definitely is a shock it happened here. It’s a quiet neighborhood. You know your neighbor on your left or your right, but people are pretty isolated,” Riley told The Post.

“I haven’t seen anyone come out. There are still all these notes on the door, so it looks like nobody has been in there.”

Washington County Sheriff Nate Brooksby previously said that Twiggs was in a “safe space very far away from St. George,” and needed “to lay low for a long time.”

Twiggs has not been charged with a crime in connection with Kirk’s shooting, and has been an integral part of the investigation as Robinson’s apparent go-to in the immediate aftermath of the assassination.

Robinson apparently left a note underneath Twiggs’ desktop keyboard and instructed him to find it after Kirk was shot.

The alleged assassin also texted Twiggs to confess to the murder, with Twiggs responding in apparent disbelief as he tried to rationalize what was quickly unfolding.

Twiggs, a transitioning transgender woman, was described as the “black sheep” of his Utah family, who were unaware of his reported relationship with Robinson.

His parents kicked him out in 2018 because his father deemed him “problematic” and disrespectful, according to a relative. The family member also alleged that Twiggs overindulged in video games and started “acting out” because of substance use.

Twiggs lived with his grandparents before moving in with a group of college roommates. After they graduated and the roommates moved out, Robinson entered the fold.

He shared support for former President Joe Biden in 2022 and joked about being targeted by advertisements for different mental health disorders, including schizophrenia and borderline personality disorder.