Oakland, California (CNN) -- A former police officer convicted of involuntary manslaughter for killing an unarmed man in Oakland, California, apologized to the public and described his memories of the moments after the shooting in a handwritten letter obtained by CNN.
In the letter, Johannes Mehserle said he was "truly sorry" for killing Oscar Grant.
"For now, and forever, I will live, breathe, sleep, and not sleep with the memory of Mr. Grant screaming 'you shot me' and me putting my hands on the bullet wound thinking the pressure would help while I kept telling him 'you'll be okay,'" Mehserle says in the letter. "I tried to tell myself that maybe this shot would not be so serious, but I recall how sick I felt when Mr. Grant stopped talking, closed his eyes and seemed to change his breathing."
Mehserle's letter was dated July 4, four days before a jury found him guilty of involuntary manslaughter for shooting 22-year-old Grant on an Oakland train platform on January 1, 2009.
Mehserle, who was on duty as a Bay Area Rapid Transit police officer at the time of the incident, said at the trial that he intended to draw and fire his Taser rather than his gun, CNN affiliate KTVU reported.
"I have and will continue to live everyday of my life knowing that Mr. Grant should not have been shot," his letter said. "No words can express how truly sorry I am."
The involuntary manslaughter conviction usually carries a maximum four-year sentence, but some in Oakland expected a tougher penalty for the former police officer, and took to the streets in protest Thursday.
Crowds broke the glass of a Foot Locker and other stores. Others threw sneakers out of the store as police wearing gas masks stormed the area.
Outside the courtroom, Grant family members expressed outrage at the verdict.
"My son was murdered. He was murdered. He was murdered. My son was murdered," said Grant's mother, Wanda Johnson. "The system has let us down, but God will never ever let us down."
Johnson and other speakers said African-Americans have been the victims of police abuse and a biased judicial system. She said Mehserle wasn't found accountable.
"We couldn't get even six hours of deliberations," said Johnson, who accused jurors of being unfair.
Oakland police said there were 78 arrests during Thursday night's protests on charges including failure to disperse, resisting arrest, burglary, vandalism and assaulting a police officer.
Mehserle's trial was moved from Alameda County to Los Angeles because of pre-trial publicity.
Members of the jury, which included no African-Americans, said they were unanimous in their decision. Their finding indicates that Mehserle was criminally negligent.
The shooting was captured on a bystander's cell-phone video camera. The video, which showed Mehserle pulling his gun and fatally shooting Grant in the back as another officer knelt on the unarmed man, was widely circulated on the Internet and on news broadcasts, and it spurred several protests in and around Oakland.
Mehserle resigned his position a few days after the incident and was later arrested in Nevada.
His sentencing is set for August 6. Involuntary manslaughter carries a maximum sentence of up to four years in prison under California law. But the judge could add an "enhancement" that could provide a longer sentence because a firearm was used.