MEXICO CITY (Reuters) - Mexico’s prison population has fallen by a fifth from the peak in 2014 as fewer people are locked up under a new criminal justice system, with experts saying that cases are falling apart under a higher bar for police conduct and standards of evidence.

The number of people imprisoned has dropped 19.8 percent from a record of 258,563 in September 2014, according to the statistics from the National Security Commission (CNS), meaning for the first time in more than 20 years there are technically more beds than inmates.

The drop in the prison population has coincided with a spike in murders to record levels. Politicians have linked rising crime to the U.S.-backed 2008 reform, introduced to modernize Mexico’s opaque and slow justice system.

It was adopted by many states by 2014 and was fully in force from mid-2016.

Data shows fewer people have been entering prison. Even justice experts who support the reform say police and prosecutors are struggling to make watertight cases due to a lack of training in the new system across the country.

“They had eight years to make the transformation and they didn’t do it,” said Maria Novoa, a criminal justice expert at think tank Mexico Evalua. “We don’t have either the institutions or trained people.”

Novoa said police are not detaining people because they fear they could make mistakes and be held liable. She also said prosecutors are overusing a rule that allows suspects to face the criminal process at liberty rather than trying to ask a judge to detain them.

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In 2016, 37 percent fewer people entered state prisons than in the prior year, according to the latest data from national statistics institute INEGI. And contrary to political rhetoric, fewer were also let out, the data shows.

Neither Mexico’s attorney general’s office nor the Federal Police returned a request for comment.

The 2008 reform moved Mexico to an adversarial justice system with oral trials from an inquisitive one mostly done on paper, with a deadline of June 2016.

It radically changed the role of the police and prosecutors, by giving police greater responsibility and raising the evidentiary standard that prosecutors must reach to get a conviction.

It also limits pre-trial detention, bringing an end to a practice where people were locked up for years without trial.

In 2017, for the first time in more than 20 years, the country has enough space for all prisoners. But their uneven distribution across the country and a number of federal inmates in state facilities mean prisons in at least 13 states still have overcrowded facilities.

Think tank CIDAC estimated last year that the reform would take 11 years to take hold properly, due to lack of police training in high quality investigations.

“We always knew that for the new system to work people would have to be trained, but there was a lot of resistance and lack of interest from state governments,” said Vicente Sanchez, a professor at the College of the Northern Border.

“Now governors and prosecutors blame the new system and want to undo it.”