A judge has ruled he cannot be sure the death of a man who had a cardiac arrest in custody was caused by the use of a restraint belt across his face.
Thomas Orchard, 32, was arrested and taken to Heavitree Road police station in Exeter, Devon, in October 2012.
During his detention, Mr Orchard, who had paranoid schizophrenia, had an emergency response belt (ERB) on his face for five minutes and two seconds.
His family said they were "dismayed" by the judge's ruling.
"We still think that Thomas's death was a needless one caused by sloppy and dangerous practices," said his mother Alison Orchard in a statement outside court.
"We have always been hopeful that Thomas's death wouldn't be in vain and that lessons would have been learned."
Last October, Devon and Cornwall Police pleaded guilty to breaking health and safety rules over the use of the ERB, which prevents spitting or biting. The force will be sentenced on 3 May.
The charge related to a failure to ensure non-employees, including Mr Orchard, were not exposed to risks in connection with the ERB.
Following a trial of issue at Bristol Crown Court to resolve a series of disputed matters, including whether the use of the belt was a contributory factor in Mr Orchard's death, Judge Julian Lambert ruled it was not possible to be sure.
He said for him to have been convinced the ERB was a significant cause of Mr Orchard's death, he would have had to be sure of the "positioning and tightening of the belt", but said " the evidence is not clear".
After his restraints were removed, he was left in a locked cell for 12 minutes before custody staff re-entered and began resuscitation.
Mr Orchard, who worked as a church caretaker, died in hospital seven days later.
In a statement, Chief Constable Shaun Sawyer said he "respected" the judge's decision which "had always been the position of Devon and Cornwall Police".