A convicted child sex offender behind bars has told cops he left a suburban Washington mall with a 10-year-old girl and her older sister the day they vanished without a trace 40 years ago.

Lloyd Welch, a “person of interest” in the disappearance of the two girls, also told cold case detectives that the next day he saw his uncle sexually assaulting one of the girls, the Washington Post reported Saturday.

The Post cited search warrant affidavits that reveal for the first time how detectives linked the 58-year-old Welch to the missing girls, Katherine Lyon, 10 and Sheila Lyon, 12. The girls were last seen having pizza at the Wheaton Plaza mall in Maryland on March 25, 1975.

The newly unsealed affidavits say detectives interviewed Welch in a Delaware prison where he is serving a 30-year sentence in connection with the sexual assault of a 10 year-old girl. He pleaded guilty to that charge in 1998. His rap sheet also shows a prior child sexual assault conviction in South Carolina in 1994.

“During these interviews, Lloyd Welch has admitted he left Wheaton Plaza in a vehicle with the Lyon sisters on the day they disappeared,” the Post quoted the affidavits as saying.

The filings say he told the detectives his uncle Richard Welch was involved in girls’ kidnapping and that another relative, a juvenile, was with them when they left the mall with the girls in a vehicle, the Post said.

The court papers go on to say that during the interviews, Welch said that after he was dropped off at his home, his uncle and the juvenile drove off with the girls.

The newspapers said that according to the affidavits Welch told the detectives that the day after the abduction he went to his uncle’s home and saw his uncle sexually abusing on of the sisters.

“Lloyd Welch claims that he left the residence and never saw the sisters again,” the affidavits say.

Authorities named Welch and his 69-year-old uncle persons of interest in the case in 2014. They have not been charged. Last December, Richard Welch’s wife was charged with lying to a Virginia grand jury that is probing the disappearance.

Last month, authorities held a press conference to announce they had been conducting a “forensic dig” for the girls’ remains on land owned by Richard Welch and his sister on a rural Virginia mountain. Bones have been found but authorities have not said if they belong to the two girls.

The affidavits say that a week after the Lyon sisters disappeared, Lloyd Welch told a Wheaton Plaza security guard that he’d seen the sisters leaving the mall in a car with a man. He repeated the story to detectives who gave him a polygraph test. The polygraph showed he was being untruthful.

The detectives wrote up a report on the interview and the polygraph results and stuck it in the file.  Cold case detectives hunting for new leads in the disappearance of the Lyon sisters found it in 2013, according to the affidavits.

Lloyd Welch has told the Post in an earlier letter that he had “nothing to do” with the disappearance of the two girls.

The paper said Richard Welch has declined comment but that his daughter has defended him. She has called the allegations against her father a lie.

The paper also tracked down the juvenile Lloyd Welch said was in the car during the kidnapping. Thomas Welch is a cousin of Lloyd Welch and was 10 ½ in 1975.

“I haven’t done anything,” he said. He also said he doesn’t think Richard Welch, his uncle, was involved in the girls disappearance but isn’t so certain about his cousin.

“If Lloyd did this, I hope he fries for it,” Thomas Welch told the Post.

Police in Montgomery County, Md., declined to comment on the affidavits. Montgomery Police Chief Tom Manger said Friday investigators are determined to find out what happened to the two girls.

"We believe that there are people, including family members of Dick and Lloyd Welch, who have information that would further this investigation," Manger said.