A cousin of the man accused of killing Sheila and Katherine Lyon in 1975 has told detectives that the girls may have been kept alive for at least five days after they were abducted from a Maryland shopping mall, according to law enforcement officials with knowledge of the case.

The cousin, Thomas Welch Jr., told detectives that on Easter 1975, when he was a young boy, he saw two blond girls at the Hyattsville, Md., home of the suspect’s uncle, Richard Welch, the officials said. The cousin told detectives he now thinks the girls were the Lyon sisters.

Reached by phone Thursday, Thomas Welch declined to comment, saying he was under a gag order and not allowed to talk about the case.

His recollections to detectives, provided eight days ago, come as police continue to aggressively investigate the 40-year-old case. It was on March 25, 1975, that Sheila Lyon, 12, and her sister Katherine, 10, vanished after walking to the Wheaton Plaza mall to look at Easter decorations and meet friends. Authorities think that the girls were killed and their bodies burned in a remote area of Virginia.

Last year, police labeled Richard Welch, 70, and his nephew “persons of interest” in the girls’ disappearance. Earlier this month, a grand jury in Virginia indicted the nephew, Lloyd Welch, 58, on two counts of first-degree murder in the girls’ deaths.

Richard Welch has not been charged and has maintained his innocence. But two weeks ago, in announcing the charges against Lloyd Welch, officials made no secret that they were continuing to investigate Richard Welch. “He remains and will be a primary focus as we move forward,” said Randy Krantz, the top prosecutor in Bedford County, Va., where forensic experts have been searching for the girls’ remains in a rugged area known as Taylors Mountain.

The law enforcement officials now say that earlier this month, Thomas Welch told detectives that he thought he’d seen the girls at his uncle’s house. It wasn’t clear whether Thomas Welch remembers seeing his uncle with the girls at the time. The sources also say that in June, a nephew of Richard Welch’s who lives in Calvert County told detectives that Richard Welch had confided in him recently that he and other family members had raped and killed the Lyon sisters and disposed of their bodies on Taylors Mountain. The officials spoke on the condition that they not be named because the case is ongoing.

Richard Welch has declined to comment, but one of his attorneys, Richard Arnold, said Thursday that his client has long said he did nothing wrong. “He has asserted his innocence to me over and over again,” Arnold said.

He described Richard Welch as someone who has trouble reading and writing and isn’t by nature aggressive. “He just is a very passive fellow,” Arnold said.

Arnold declined to talk specifically about the recent allegations from the two Welch cousins. But he questioned how one of them could suddenly recall information after all these years. He noted that other Welch family members are recalling events from many years back.

“I wonder if they took a class for that — to remember something after not remembering it for 40 years,” Arnold said.

Patricia Ann Welch, Richard Welch’s daughter, said that her father is innocent, and that other family members are making incorrect statements to detectives to protect their deceased relatives. She said that she was 8 in 1975 and would have known if there were suddenly two young girls in her house — and there weren’t. “It’s getting outrageous and disturbing,” she said of the investigation.

“He’s sick over it,” Welch said about her father. “He can’t believe his family members are putting him through this. . . . If these people’s mothers knew what they were saying about my dad, they’d be rolling over in their graves.”

Prosecutors have acknowledged that it is difficult to prove a decades-old murder case, particularly one in which the bodies are never found. Their case so far is built in large part on recollections — some fuzzy, some contradictory — from members of the Welch family. Detectives have also searched homes in Maryland, Virginia and other states and gathered letters and electronic communications among Welch family members.

“The information that investigators are gathering is good, quality information,” Capt. Paul Starks, a Montgomery County police spokesman, said Thursday. “We continue to move forward on this case.”

For decades, the disappearance of the Lyon sisters has remained one of the Washington area’s most heart-wrenching mysteries.

Various Montgomery police detectives worked on the case, and in 2013 a group of them going through old files found reason to start looking at Lloyd Welch. He had been at the Wheaton mall the day the Lyon sisters disappeared, but detectives at the time appeared to have classified him as an unreliable witness rather than a suspect.

Welch went on to live in other states and was convicted in South Carolina and Delaware of sexually assaulting 10-year-old girls. Detectives found him incarcerated for one of those crimes, in Delaware, and in 2013 went to interview him.

The first conversation lasted eight hours, according to court records. At one point, Lloyd Welch told detectives he thought the Lyon sisters had been “abducted, raped and burned up,” according to a search warrant affidavit filed this year in the case in a Virginia court. In that initial interview, Lloyd Welch eventually told detectives that he didn’t want to answer any more questions and advised detectives to charge him with murder and leave him in prison, according to the affidavit.

Lloyd Welch later implicated other family members, telling detectives that on March 25, 1975, he left the mall in a car with the Lyon sisters, his uncle Richard Welch and cousin Thomas Welch.

More recently, according to several officials, Lloyd Welch has implicated his own father, also named Lloyd Lee Welch, now deceased, who in 1975 lived in a house on Baltimore Avenue in Hyattsville, about 10 miles from the mall. According to the officials, Lloyd Welch recently said he saw his father and his uncle, Richard Welch, kill one of the Lyon sisters.

In May, detectives searched a basement at the Baltimore Avenue home, looking for evidence. It is not clear what, if anything, they found.

Although Thomas Welch declined to comment Thursday, he said in an earlier interview that Lloyd Welch was lying and that he did not leave the mall with the Lyon sisters.“I haven’t done anything,” he said.

In that earlier interview, Thomas Welch said if Richard Welch was involved, he should be prosecuted. But he also said, “I still don’t feel in my heart that my uncle had anything to do with this.”

But according to the officials, Thomas Welch said he saw two girls resembling the Lyon sisters at his uncle’s home on the Easter after they went to the mall. The officials wouldn’t say whether Thomas Welch told investigators he saw Richard Welch at the home on that day. But Thomas Welch did say, according to the officials, that he saw Lloyd Welch with the girls.

Jennifer Jenkins contributed to this report.