Police have indicted a convicted sex offender with two counts of first-degree murder in the disappearance of two Maryland sisters 40 years ago.

Lloyd Lee "Michael" Welch Jr. was a person of interest in the disappearance of 12-year-old Katherine Lyon and 10-year-old Sheila Lyon, who were last seen at a Maryland shopping mall on March 25, 1975.

Welch is in a Delaware prison now, after he pleaded guilty to sex abuse charges in Delaware and South Carolina.

Welch was indicted Friday. The two charges are first-degree felony murder "while intending to defile," and they carry the possibility of the death penalty. Welch is expected to be extradited in the next 30 days.

He was charged by a grand jury in Bedford County, Virginia, where many law enforcement agencies have been searching Taylor Mountain for any evidence linked to the girls. That search area included land that had been owned by Welch's family.

Prosecutors would not say whether they located anything in the search, but they said they were prepared to prosecute "a no-body case."

“Typically in a homicide you have an autopsy and you have a body and you can prove what the cause and mechanism of death is,” Montgomery County State's Attorney John McCarthy said. “In a no-body murder case you have to prove that they are in fact dead and you have to prove the agent by which they died.”

The indictment was unsealed Wednesday. The news was a big development in a case that terrified many families in the suburbs of Washington, D.C., leading families to keep their children inside and supervise them every time they went out.

Montgomery County Police said they spent about 16,000 hours of investigation into the Lyon sisters’ case in the past two years, serving 50 search warrants in recent years and interviewing or interrogating about 100 people. Bedford County Sheriff Mike Brown said his team has spent 1,500 to 2,000 hours on the case since the investigation led to the county in September 2012.

Three generations of the Lyon family were present at the news conference. McCarthy said they "did not choose to be victims" and "beg you for their privacy."

"If you have anyone in Montgomery County, they can tell you a personal story about how they or their family was impacted," said Montgomery County Police Chief J. Thomas Manger at the news conference.

Manger said he was in college when the girls disappeared and vividly remembers his mother talking about the case. McCarthy said he was a young teacher at the time.

"This case has never left the collective consciousness of this police department nor our community," McCarthy said.

Montgomery County police are crediting a young detective on the county's cold case division with connecting a witness sketch with a mug shot of Welch from the 1970s.

Police have said Welch is originally from the D.C. area and was known to hitchhike throughout the region. Between the 1970s and the mid-1990s, he traveled extensively through the United States while working for a carnival company with his girlfriend, Helen Craver, police said.

His uncle, Richard Welch, was named a person of interest in October. Authorities believe he was a security guard in the Wheaton area at the time of the sisters' disappearance and that the family owned property in Bedford County, said Montgomery County Police Chief Russ Hamill last October.

Police have also charged 65-year-old Patricia Jean Welch of Hyattsville, Maryland — Richard Welch's wife — of perjury in the case for lying to a grand jury.

On March 25, 1975, the Lyon sisters had planned a day at Wheaton Plaza, now known as Westfield Wheaton Mall. They were on spring break and wanted to get pizza for lunch and see the Easter decorations at the shopping center. The sisters were seen by multiple there, including one of their brothers, McCarthy said.

With less than $4, they left their home in Kensington, Maryland, and walked the half-mile or so to Wheaton Plaza.

There, a friend saw the girls outside the Orange Bowl restaurant with an older man who had a tape recorder and a briefcase, according to news and missing persons reports.

The girls were later spotted walking home, but by their 4 p.m. curfew, they hadn't arrived. By 7 p.m. that night, police had been called.

Tips flowed in, but to no avail. Sheila and Katherine were never seen again.

On April 1, 1975, Lloyd Welch told a Wheaton Plaza security guard he'd seen the Lyon sisters get in a car with an unknown man and leave the parking lot the last day they were seen, according to court documents. Police interviewed Welch, and a polygraph deemed he wasn't telling the truth.

Cold case investigators discovered the report of Welch's interview with police in May 2013. Since then, Welch has told investigators his cousin, Thomas Welch Jr., and uncle, Richard Welch, were involved in the kidnapping, according to court documents. Lloyd Welch said they dropped him off near his home and continued on with the Lyon sisters. He said the next day he went to Richard Welch's home and saw him sexually abusing with one of the sisters. He said he left and never saw the girls again.

In his interviews with investigators, Lloyd Welch repeatedly said he had nothing to do with the girls' deaths and does not know where they are buried, according to court documents. But since he was named a person of interest in February 2014, another inmate in his cellblock told prison officials Welch told him about burying them on a family property in Virginia.

On Sept. 11, investigators found about 30 graves on Taylor Mountain, according to court documents. Only one headstone was found, reading "Martha D. Overstreet Parker, September 24, 1866-April 8, 1906." The next day, a niece of Richard Welch was told the property was being secured as a crime scene.

In December, investigators interviewed a man who said he met Welch on Taylor Mountain in 1975 and helped him get two red-stained, Army-style duffle bags out of the trunk of a car, according to court documents. They were thrown on a fire. The man said they weighed about 60-70 pounds and smelled like death.