The murder charges filed against the Baltimore officers who arrested Freddie Gray could be dropped, because the police investigation into his death doesn't support the prosecution's case, it's been reported.

Last Friday, Baltimore City State's Attorney Marilyn Mosby levelled charges ranging from assault to second-degree murder at six police officers involved in Gray's death. He had suffered a fatal spinal injury while in police custody.

However, officials familiar with the case have revealed that police investigators do not agree with the charges.

The murder charges filed against the Baltimore officers who arrested Freddie Gray could be dropped

The officials said that the internal probe team do not believe a charge more serious than manslaughter should be brought against any of the officers, according to CNN.

What's more, defense lawyers are mounting a challenge to Mosby's assertion that the officers had unlawfully arrested Gray because the knife he had in his pocket is considered legal under Maryland state law.

Marc Zayon, the attorney for Edward Nero, one of the officers charged, argued in a motion filed Monday that the knife in Gray's pocket — described in charging documents as 'a spring assisted, one hand operated knife' — is in fact illegal under state law.

Charged: Caesar Goodson (left) and Garrett E. Miller (right) were among the officers arrested on Friday

Accused: Brian W. Rice and Alicia D. White are also facing charges stemming from Freddie Gray's fatal arrest

Arrested: William G. Porter (left) and Edward M. Nero (right) were also taken into custody

'If the facts were that the knife was illegal then the Gray arrest would be justified. Even if it wasn't illegal and the officers acted in good faith, it would be the same result. All charges fail,' said lawyer Andy Alperstein, who is not involved in the investigation.

Defense lawyers may also exploit the past of a member of Mosby's investigative team, CNN said.

One of her lead investigators is Avon Mackel, a former senior Baltimore police officer whose reputation is tainted by a 2009 incident that led to him being removed from his command post.

Swift: Thousands praised Marilyn Mosby, Baltimore's young top prosecutor, for quickly moving forward with charges against the police officers they see as responsible for the death of Freddie Gray

He was accused of not tackling two officers who failed to report a robbery.

Lawyers for the defense could reason that he holds a grudge against the department.

Nero and Officer Garrett Miller are charged with misdemeanors. Four others — Sgt. Alicia White, Lt. Brian Rice and officers Caesar Goodson and William Porter — are charged with felonies ranging from manslaughter to second-degree 'depraved-heart' murder.

An official told CNN: 'If this case falls apart, then does Baltimore burn?'

The charges against the officers came near the close of a turbulent week in which violence, looting and fires erupted in the streets April 27 only hours after Gray's funeral that Monday.

Meanwhile, Baltimore's mayor, Stephanie Rawlings-Blake has asked the US Justice Department to investigate the case.

The Democratic mayor now says she'll accept outside intervention to rebuild public trust in a city torn by riots over the death of Freddie Gray.

'I am determined not to allow a small handful of bad actors to tarnish the reputation of the overwhelming majority of police officers who are acting with honor and distinction,' she wrote in a letter to the new U.S. attorney general, Loretta Lynch.