The FBI says Lloyd Lee Welch, also known as Mike Welch -- who is serving time in a Delaware prison for sexual assault -- was at the shopping mall March 25, 1975, when Shelia and Katherine Lyon vanished without a trace. The girls were 12 and 10 respectively at the time of their disappearance.
Officers are not calling Welch, 57, a suspect, but released his photo on Tuesday and asked members of the public if they know anything about his past, CNN said.
Many witnesses said they saw the girls at the mall and walking home, but the girls never arrived home that day.
Anonymous callers asked the family for money in return for the girls, but one picked up the cash, and the calls and tips ended.
WUSA-TV, Washington, said Welch has a long criminal history and has been charged in multiple states with sex offenses after traveling the country as a carnival worker from the 1970s to the 1990s.