Los Angeles - The death sentence of the "Grim Sleeper" serial killer last week put to rest a case that spanned more than three decades, but it left another mystery wide open.Police are still trying to identify 33 women whose snapshots were found in Lonnie Franklin jnr's home after his arrest.The images were part of a chilling discovery of nearly 1 000 photos of women or teenage girls - many nude and some who appeared to be unconscious or dead - hidden in Franklin's house. The collection included photographs of several victims, which leads police and prosecutors to believe Franklin left behind many more.Detective Daryn Dupree said Franklin is one of the most prolific killers and could have killed as many as 25 women from the late 1970s until his arrest in 2010. That includes the period of 1988 to 2002 when police originally thought the killer took a break - an apparent hiatus that helped coin his nickname."I don't think he stopped because he was getting away with it," Dupree said. "I think he slowed down, but I don't think that big gap was as much as we thought it was."Franklin, 63, was sentenced to die on Wednesday for murdering nine women and a 15-year-old girl in South Los Angeles. Prosecutors outlined evidence of three additional slayings he wasn't charged with and two other women who went missing and they suspect he killed.The student identification card of Ayellah Marshall, 18, who disappeared in 2006, was found in Franklin's garage along with the Nevada driver's license of Rolenia Morris, 29, who was last seen in 2005. Two snapshots of Morris were found in his photo collection.Mystery manDefence attorney Seymour Amster wouldn't comment on the possibility that his client was involved in other killings. Franklin denied any role in the killings to investigators, and his attorneys had suggested a mystery man was the real killer.Many of the slayings occurred when US cities were reeling from the crack cocaine epidemic. Franklin targeted young women in the poor area where he lived. Some were drug users who had turned to prostitution in desperation to support their addiction."Back then, you'd drive down the street and girls would be trying to jump in your car," said Dupree, who grew up in the area and witnessed the crack's effect.Several other serial killers prowled the area, preying on the same type of victims, who were often sexually assaulted and then dumped in alleys, parks or trash bins.The killings were believed to be the work of one man dubbed the "Southside Slayer," though several culprits were later arrested and charged with additional crimes as DNA evidence became more sophisticated.The killings inspired the formation of a community group, the Black Coalition Fighting Back Serial Murders, to raise awareness of the danger to women and pressure police to investigate more thoroughly.200 missing womenOne of the group's founders, Margaret Prescod, continues on that mission. She distributes fliers with the photos of the unidentified women found in Franklin's house and said she wants to find out what happened to about 200 women who have gone missing in that area or whose killings remain unsolved."We're glad there's a killer off the street, but that doesn't mean everything is resolved," she said. "There is a whole set of women and nobody knows what happened to them."She is hopeful that the women in those photos turn out to be alive. Police originally reached out for the public's help identifying 166 women. It's been narrowed to 33 faces.Dupree said officers get inundated with calls every time there's a story about the photos. He thinks another case will eventually connect back to Franklin as more DNA from older cases gets added to a databank.Deputy District Attorney Beth Silverman, who put Franklin and two other serial killers in the area behind bars, is not so sure.For one thing, she said, the task force that finally caught Franklin screened 400 cold cases. Secondly, Franklin, a former garbage collector, disposed of many of his bodies in dumpsters."Knowing he worked for the city as a sanitation truck driver and had access to the landfill, who knows how many bodies were taken away," she said.One victim, Janecia Peters, could have ended up on the way to the dump if a homeless person looking for cans hadn't noticed her red fingernails poking through a trash bag on New Year's Day 2007.When DNA on Peters' body connected her to previous cases, Police Chief William Bratton created the task force that re-examined all the murders and eventually arrested Franklin.