London ? Police investigating the murders of five prostitutes in a town in eastern England said Saturday they have no formal suspects but are focusing on 50 to 100 people they want to question.

Police also released closed-circuit television footage from the train where one victim, 24-year-old Anneli Alderton, was last seen Dec. 3 – the date of the footage – in an effort to track her movements before her body was found seven days later.

More than 350 officers from forces across Britain are assisting the Suffolk police with the investigation into the killings, which have raised fears for the safety of women around Ipswich. The naked bodies of all five women were found over 10 days.

Assistant Chief Constable Jacqui Cheer of the Suffolk county police said investigators have a list of people to question.

“We have between 50 and 100 people that we’re interested in,” Cheer told the British Broadcasting Corp.

“A good way to describe the progress we are making would be to compare the investigation to a jigsaw,” she added. “We have constructed the edges. Now we have to fill in the middle.”