From left: Derek Howard, John Raposo, and George Billadeau reacted after they were acquitted Monday in the 2009 death of patient Joshua Messier. Matthew J. Lee/Globe Staff/Globe Staff

Three former prison guards at Bridgewater State Hospital were acquitted Monday of involuntary manslaughter charges in the 2009 death of Joshua K. Messier, a 23-year-old patient with schizophrenia they forcibly strapped to a bed in a brutal scene captured on prison video.

“Retrospectively, the conduct of these three defendants left much to be desired, but does not constitute wanton and reckless conduct,” said Superior Court Judge Jeffrey A. Locke, who found the men not guilty.

Messier’s death led to sweeping reforms at the state’s psychiatric hospital, and the verdict capped a long-running legal saga spurred by a Globe expose and a special prosecutor’s investigation.

The former guards embraced their family members after the verdict. Defense lawyers expressed sympathy for Messier and his family while offering harsh criticism of the Department of Correction for failing to train the guards in the use of four-point restraints or enforcing a department rule that called for a medical assessment of each patient before he was placed in four-point restraints.

“There was institution-wide negligence here,” said defense lawyer Edward P. Harrington.

Lisa Brown, Messier’s mother, said she had expected a guilty verdict, yet she praised Locke for his decision.

“I think he did what he thought was right. I just don’t agree with it,” she said. “I take solace from the fact that because of Joshua’s death, many things have already changed in the mental health care system. There are no more prison guards inside Bridgewater State Hospital.”

Messier’s mother, Lisa Brown, reacted as the verdict was handed down. Matthew J. Lee/Globe Staff

The verdict came after a five-day nonjury trial in which Locke heard evidence that the May 4, 2009, fatal episode began when Messier suffered a psychotic attack and punched a guard. After a fistfight, a half-dozen guards subdued Messier and took him to the hospital’s Intensive Treatment Unit, where he died after being strapped down to a bed by his wrists and ankles.

Lawyers for the defendants attributed Locke’s verdict to doubts raised about the quality of video evidence, the inadequate training the guards received, and a wide range of factors that may have contributed to Messier’s death other than the guards’ actions. Among them were the presence in Messier’s system of toxic levels of a psychiatric medication known to present a risk of cardiac arrest; Messier’s obesity – a side effect of his medications; and a head injury he sustained in a fight with guards.

“There was reasonable doubt everywhere you looked in this case,” said Kenneth H. Anderson, one of the three defense lawyers.

Instead of formal training, guards learned restraint training “on the job,” the lawyers said, and typically completed the medical assessment only after the patient had been restrained. After Messier’s death, Bridgewater officials told all command officers that placing a patient into four-point restraints before being examined by a medical clinician was no longer an option.

“We take no solace in this verdict,” said defense attorney Hugh R. Curran. “Our hearts go out to Joshua Messier and his family. As the judge said, it was a perfect storm of circumstances that contributed to Joshua’s death. There are no winners today.”

Martin F. Murphy, the special prosecutor in the case, said he believed the lack of adequate training and the department’s failure to follow its own regulations on restraints led to the acquittal.

“The fact that these guards were not as well trained as they should have been and the fact the Department of Correction didn’t abide by its own policy made it very difficult to find any possible set of individuals, including these three, personally responsible for what happened to Joshua,” Murphy said.

A larger obstacle might have been the poor quality of video evidence that showed Messier being forcibly placed into four-point restraints by about a half-dozen guards.

“The video is of insufficient quality and clarity to permit a full understanding of Joshua’s movements and position at critical times,” Locke said.

Locke also noted that state medical examiner Mindy J. Hull found that, in addition to the psychiatric medication Clozapine that Messier was taking, several preexisting conditions may have played a role in causing him to develop an irregular heartbeat, which probably led to his cardiac arrest and death.

The conditions included his paranoid schizophrenia, which includes a risk of sudden death, high blood pressure, and a fatty liver, which may have prevented his body from breaking down the Clozapine, Hull found.

The video included a crucial 54-second segment in which prison guard Derek Howard appears to put his full weight on Messier’s back, while Messier has his hands cuffed behind him. Howard then appears to bend Messier over at the waist until his chest is pushed toward his knees, a maneuver that could have made it difficult for him to breathe and induced the irregular heartbeat that led to his death.

For about 40 of those seconds, a second prison guard, John C. Raposo, appears to press down on Howard’s back, seemingly increasing the pressure on Messier. Meanwhile, the third guard charged in Messier’s death, supervisor George A. Billadeau, stands apart, watching events unfold.

Plymouth District Attorney Timothy J. Cruz initially declined to prosecute the guards, saying there was insufficient evidence of a crime.

But, after the Globe reported in 2014 about the guards’ rough treatment of Messier, then-Attorney General Martha Coakley appointed special prosecutor Murphy to investigate. Murphy requested a judicial inquest where District Judge Mark S. Coven found that evidence in the case could support a finding that the guards were criminally liable. A grand jury later indicted the guards on charges of involuntary manslaughter and criminal civil rights violations.

The guards were also fired after the Globe’s report.

On Monday, Cruz said, “There was no new evidence present at this trial that our office had not previously pored over as part of our investigation . . . today, Judge Locke came to the same determination that there was insufficient evidence to support any charges against these three men.”

Locke noted that the Department of Correction has made significant changes at Bridgewater State Hospital to prevent similar incidents.

“Hindsight is 20-20,” Locke said. He noted that the department has revamped its training in the use of four-point restraints and “has put mental health professionals rather than correctional officers in charge of operations at Bridgewater.”

Those changes went into effect in April, when the private firm Correct Care began providing medical and mental health care at Bridgewater, closing the Intensive Treatment Unit where Messier died and replacing prison guards with a security force trained in handling men with serious mental illness.

“I do believe that things are going to keep changing,” said Brown, Messier’s mother. “I have to believe that.”

Still, Brown said she never expected the guards to be found not guilty.

“With my whole heart, I thought they were going to be found guilty,” she said. “The reason is common sense. You don’t need training to know that what they did to my son was going to cause serious harm.”