A former Bay Area Rapid Transit police officer was found guilty on Thursday of the "involuntary manslaughter" of an unarmed black male who was laying handcuffed before him on the ground.

At the trial, former officer Johannes Mehserle "told jurors that he heard a pop and thought the Taser had malfunctioned when 22-year-old Oscar Grant was shot to death on New Year's Day 2009," Los Angeles news channel KTLA reported.

The killing was captured on video by several individuals who'd boarded a nearby train.

"For the life of me, I cannot accept that this was an involuntary manslaughter," John Burris, an attorney for Grant's family, said after the verdict was announced. He called the jury's decision "a compromise verdict" but noted that he'd never litigated a case that saw an officer convicted on any such charge.

He called the verdict "a very small step forward, but also a step backward as well."

The jury's deliberation lasted less than seven hours.

The killing sparked a series of rowdy and sometimes violent protests, even as Grant's family appealed for calm.

After Thursday's verdict, Mehserle could get four years in prison, "plus added time for using a gun," according to Mercury News. He had been free on $3 million bail, but the judge ordered Mehserle taken into custody after the verdict was read.

Burris suggested that the gun attachment could add years to that sentence, up to a maximum of 12 if the judge is sympathetic to the victim impact statements. But, he added, "he may very well get a sentence that does not require him to go to jail."

Grant's uncle Cephus Johnson called the decision "a slap in the face" to the family and justice itself, and Grant's mother repeated the word "murder" five times in an emotional, public reaction.

While the attorney pleaded on behalf of the family for calm in the streets, Johnson said he could not control the actions of their community, which is likely to "express their feelings in their own way." He further appealed for peace.

"It is my concern that those agitators, those policemen in civilian uniform, could be" responsible for encouraging violence, Johnson added.

The judge has ordered a probation report to be prepared. Unconfirmed reports claimed that area police departments had SWAT teams at the ready.

The family will have a chance to make victim impact statements to the court on August 6, at which point the judge will determine sentencing.

Pictured above: Oscar Grant.

Story was updated to correct information on Mehserle's potential jail sentence.