Ciudad Juarez, Mexico ? State prosecutors have long said that 88 women found dead in the desert outside this rough border city were killed by a gang of criminals working with bus drivers. But few people in Ciudad Juarez believed them, least of all the mothers of the victims.

Now, the federal government has formally declined activists’ requests to take over the state of Chihuahua’s decadelong investigation, saying they also believe there is little evidence of a conspiracy.

Under Mexican law, federal investigators can only take over murder investigations if they involve federal offenses like drugs, weapons possession, organized crime and criminal conspiracy.

Federal Assistant Atty. Gen. Carlos Vega said there was nothing to support the state of Chihuahua’s theory that an Egyptian-born chemist, gang members and bus drivers planned the murders.

“We didn’t see anything to support that,” he said.

The decision was a blow to lawmakers, relatives of victims and other activists who have long criticized state investigators as inept. Even state investigators admit they need help and have openly turned to the expertise of FBI officials.

After reviewing 30 case files, federal officials announced Thursday they found no evidence to substantiate the state’s theory that more than one person is involved, and they refused to take over the cases.

The bodies of at least 88 young women — mainly slender, long-haired, and between the ages of 15 and 20 — have turned up in the desert in the past decade. All appear to have been raped, and had either been strangled or had their necks broken.

Several suspects have been arrested in the killings, but there has been just one conviction. Egyptian-born chemist Abdel Latif Sharif was found guilty in one of the earliest murders and sentenced to 20 years in prison.

Under the conspiracy theory, Sharif allegedly paid members of a gang known as the Rebels to continue the killings after he was arrested to show someone else was the killer. However, no evidence of such payments has surfaced. Other Rebel members then allegedly contacted a group of bus drivers and asked them to carry on killing.

All of the suspects arrested in the cases allegedly had some connection with Sharif or the Rebels, and almost all have said they were tortured into signing confessions. Many have been held in jail for as long as six years pending trial, even as the bodies of young women continue to turn up in the desert.

Young boys play near a set of crosses that commemorate the spot where the bodies of eight young women were found in 2001 in Ciudad Juarez, Mexico. After 10 years in which the bodies of at least 88 women have been found in the desert around Ciudad Juarez, prosecutors here are turning to the United States for help, and residents are taking matters into their own hands.

Despite the federal decision, local prosecutors said they would continue the probe.

“It is very probable that the latest murders are the work of a new wing of the very same group,” said Chihuahua state special prosecutor Angela Talavera, referring to the murder of three girls whose bodies were found February near a gravel pit.

Talavera conceded the conspiracy theory may sound bizarre, but she said: “In this case, the things that appear most unlikely, almost taken from a movie script, turn out to be true.”

She said the federal prosecutors’ objections to the conspiracy theory “may be just an excuse.”

The desperation has reached such a pitch that prosecutors have asked the FBI to help out on the case, in part because people trust U.S. police more.