( CNN ) -- The two little girls vanished nearly 40 years ago , their disappearance long faded from the public consciousness , their names etched in a stone marker their hope-sapped parents placed in a local cemetery . But Maryland police have been tenacious about solving this cold case . And on Tuesday they breathed new life into it , when they announced a possible breakthrough . Convicted sex offender Lloyd Lee Welch Jr. , 57 , who 's been serving a prison sentence in Delaware since 1997 , was near Sheila and Katherine Lyon on that March day in 1975 when they went missing , Montgomery County Police Chief J. Thomas Manger told reporters Tuesday . Welch was at a plaza that was among the last places that the two girls were seen alive , and he was observed paying attention to the sisters , Manger said . He declined to state how police obtained this lead . Manger said that anyone who has information about Welch should call authorities . Police released photographs of him and his now-deceased girlfriend , Helen Craver , about whom police are also seeking information . `` We need to know more about his activities , '' Manger said . Welch , who also went by the name of Michael Welch or Mike Welch , worked as a ride operator for a carnival company that set up their attractions in malls around the country , Manger said . His girlfriend also worked at the carnival company . Welch has `` multiple convictions for sex offenses against young girls , '' Manger said . Investigators believe Welch may have been involved in other unsolved crimes around the country . He was also a drifter and sometimes stayed in homeless shelters , Manger said . Montgomery County police have stopped short of calling him a suspect in the two sisters ' disappearance . The case faded from the headlines years ago . If they 're alive , the women would be 48 and 50 years old now . After years of holding out hope , their parents gave up . Sixteen years ago , they set a marker for their daughters in a cemetery with their dates of birth and the date they went missing , the Washington Post reported . But the police department has not forgotten Sheila , who was 12 , and Katherine , who was 10 , the day their older brother , Jay Lyon , saw them last . Maybe , in part , that 's because he joined the police force as a homicide detective just a few years after that . Taken : The coldest case ever solved . Walk to pizza parlor . The girls must have been full of anticipation when they decided to step out for some fun in their neighborhood not far from Washington , D.C. Their birthday dates were just one day apart , and celebrations were coming up in just a few days . It was also the first day of Easter school vacation . They walked half a mile to a nearby strip mall to check out the Easter decorations , the Montgomery Gazette reported . Their mother told them to be home by 4 p.m. At 7 p.m. , police received a call reporting them missing , the privately run missing persons website CharleyProject.org reported . Police swarmed out looking for them and questioned passersby . A medley of witnesses said they 'd seen the girls . One saw them walking to the shopping center . Another -- a child -- saw them talking to a stranger , a man holding a microphone and a briefcase . Police drew a composite of him based on the witness 's description . But the girls walked away from him . An hour later , their brother Jay , 15 at the time , spotted them at the shopping center eating pizza . And after that , someone else saw them walking in the direction of their home , according to the CharleyProject description . Had they arrived , they would have made it home on time . Back then , it was common for children to walk to places in the neighborhood without their parents . But when the girls disappeared , that feeling of safety was shattered , the Gazette wrote . Now-dead convict likely linked to Florida woman 's '89 disappearance . Celebrity case . The girls ' father , John Lyon , was a well-known radio host in Bethesda at the time , so the story of his daughters ' disappearance made the rounds . Anonymous callers rang the family demanding money , CharleyProject.org claims . One of them demanded their father deposit $ 10,000 in a particular location . He complied . No one picked up the cash . The man called again , but when asked for evidence that he actually had the girls , he broke off contact . In the ensuing years , the tips faded . The case languished . Until Tuesday . Arrest in 40-year-cold murder case . Senior couple charged in decades old cold cases . Oklahoma deputies pull bodies from lake , may date to '60s , 1970 disappearances . Police : Evidence in killing of former beauty queen points to ex-priest . CNN 's Michael Martinez and Dave Alsup contributed to this report .
NEW : Police ask public for information about convicted sex offender Sheila Lyon Lloyd Lee Welch Jr. was A few years later , her sister Katherine 10 , when they vanished . Their brother , Lloyd Lee Welch Jr. , saw them eating pizza that day . 12 , he joined the police force .