On Oct. 16, 2013, two detectives from Montgomery County, Md., sat down for the first time with a Delaware prison inmate named Lloyd Lee Welch. Their hope: That he had answers to the nearly 40-year-old mystery of what happened to Sheila and Katherine Lyon, two young sisters who vanished from a shopping mall in Wheaton in 1975.

When they were done, Welch had spoken for eight hours — enough to yield a 232-page transcript of the interview, according to police and court records in the case. In the interview, detectives allege, Welch acknowledged that he was at the mall the day the girls disappeared, said he believed that they’d been “abducted, raped and burned up” and that the detectives should just charge him with murder and take him back to prison.

Now, as prosecutors prepare to try Welch, 59, on murder charges here — where police contend the girls’ bodies were disposed of — the length of that first Welch transcript is just one example of the thousands of documents Welch’s defense attorneys are having to review. On Tuesday, they argued for a delay in his trial, saying the proceeding would need to be pushed to April of next year to give them time to prepare.

“It’s going to take that long for us to process, catalogue, review and conduct our investigation,” one of Welch’s attorneys, Aaron Houchens, told Judge James Updike on Tuesday.

Commonwealth’s Attorney Wes Nance said he couldn’t in good conscience argue with Houchens’s request. Updike set the new trial date.

Lloyd Lee Welch (Courtesy of Bedford County Sheriff's Office)

Nance said his office is working with Welch’s attorneys to try to get them documents in the case, including those related to other “persons of interest” developed over the decades of the case.

Nance also acknowledged that case files on those individuals are “potentially exculpatory in nature” for Welch.

The slow-going in the prosecution of Welch isn’t surprising, given the complexity and longevity of the case. For nearly four decades, the disappearance of Sheila and Katherine have remained one of the Washington region’s most heart-rending mysteries. At the Montgomery County police department, a series of investigators looked into various suspects but didn’t file charges.

In mid-2013, detectives began to take a new look at Welch, a former carnival worker whose name was in old case files.

In that early phase of the case, Welch had been considered an unreliable witness, not a suspect. That view began to change the more the new detectives looked into him and in no small measure because of where they found him: in a Delaware prison serving a long sentence for sexually assaulting a young girl.

They began talking to him in October 2013, returning again eight more times, according to court records. At least twice, he was given a polygraph. In all, more than 10 detectives from various agencies talked with Welch.

Detectives came to believe that Welch and others abducted the Lyon sisters from Wheaton Plaza mall, killed them and took their bodies to a rural mountain in Bedford County, about 200 miles southwest of Washington.

Prosecutors convened a grand jury in Bedford to further investigate the case. That produced another 40 witness statements.

Welch was indicted on two counts of murder in 2015. Law enforcement officials have consistently said that others were involved, but it is unclear if they are alive and whether anyone else will be charged.

In court Tuesday, Welch said little other than “Yes, sir, your honor” when Updike asked him if he was okay with the trial being delayed. He wore a bright-orange prison jumpsuit and was allowed to sit at the defense table without handcuffs, but his ankles were shackled.

In documents filed ahead of the hearing, Welch attorney Houchens ticked off the volume of paper and electronic documents that have been given to the defense attorneys: More than 1,900 pages of transcripts from the Welch interviews, more than 29,000 “electronic files” that cover wire-tap information, interview notes, audio recordings and video recordings, and an additional 1,600 PDFs that go back to 1975.

“It is an incredible amount of information,” said Carter Garrett, a defense attorney who represents Welch’s uncle, Richard Welch, who was cited as a “person of interest” in the case in 2014 but is not charged.

In court papers filed ahead of Tuesday’s hearings, Welch’s attorneys said they were expecting another cache of material — going back to the initial days in the case.

“This production contains over 40 years of files concerning this investigation,” the attorneys wrote. “Defense counsel has been advised that this production will be larger than the first production, and will contain approximately 10 bankers boxes of files.”

Garrett, the attorney for Richard Welch, said Lloyd Welch’s attorneys and their experts have a duty to review all material closely. “You’ve got to go through every bit of it,” he said.

In statements to investigators, Welch said that he and Richard Welch kidnapped the Lyon sisters, according to search warrant applications from investigators that had previously been filed in court. Lloyd Welch also said he later saw the girls at Richard Welch’s house, the statements to support search warrants state.

Garrett said that the new filings entered before Tuesday’s hearing shed favorable light on his client by confirming how much effort was put into solving the case.

“It appears to have been an incredibly thorough investigation,” he said, “yet my man hasn’t been elevated from person of interest to suspect. You get the feeling that Richard is telling the truth when he says he had nothing to do with it.”

Garrett said that even if prosecutors cannot get extensive corroboration of what Lloyd Welch has said, they may not need much — given that he appears to have implicated himself.

Speaking of some of his past clients, he quipped: “I’ve always said I would have won a lot more cases if it wasn’t for that nagging confession.”