Here is what we know:

On Jan. 1, 2009, BART police officers detained Oscar Grant III and a group of his friends on the Fruitvale train platform.

Grant, who had been in a fight on the train with another passenger, was on his stomach. The officers were attempting to handcuff him.

For reasons that only he will ever know, Johannes Mehserle abruptly drew his service weapon and shot the 22-year-old Hayward man in the back. Several witnesses captured the act on their cell phones.

The shooting violated police procedures for use of deadly force. Investigations by both BART and the Alameda County District Attorney's Office found that Mehserle had no justifiable reason for shooting Grant.

Mehserle's defense is that he made a mistake, that he accidentally drew his gun instead of his Taser.

But mistake or no, the bullet can't be pulled back.

A young man is dead. A mother has lost her son. A little girl will grow up without a father.

A Los Angeles jury found the former BART police officer guilty of involuntary manslaughter.

Given the gravity of the crime, Mehserle's brief sentence is a gift.

Last month, Los Angeles County Superior Court Judge Robert Perry sentenced Mehserle to two years in prison and gave him credit for 292 days served. That meant Mehserle would likely spend a maximum of seven more months in jail.

Altogether, he will serve about one year for shooting an unarmed man in the back.

It was nearly the most lenient sentence that Perry could have issued. Grant's loved ones and others understandably feel that two years in jail is not sufficient punishment for the criminal taking of a life by a police officer sworn to uphold the law.

Mehserle ought to consider himself fortunate. If he had been found guilty of voluntary manslaughter, he could have gotten as much as 11 years in prison.

Yet Mehserle's attorney Michael Rains is arguing that the former BART officer shouldn't serve any time. Rains will attempt to have Mehserle's conviction overturned. He insists that his client made a mistake, but that his actions weren't criminal.

On Friday, Rains will ask Perry to release Mehserle on bail.

The judge will consider three criteria in deciding whether to grant the request.

The defense must establish that Mehserle does not pose a threat to society, that he is not a flight risk and that there is compelling reason to believe that an appeals court will overturn Mehserle's conviction.

There is nothing to suggest that Mehserle would harm others. It is also unlikely that he will flee. Mehserle left California for Nevada after receiving death threats. However, officials at the Alameda County District Attorney's Office have said that they always knew his whereabouts. They had no difficulty locating Mehserle in Nevada, where he was finally arrested two weeks after shooting Grant.

However, the defense claim that Mehserle's conviction is likely to be overturned is without merit.

Perry threw out the problematic gun enhancement that the jurors had inappropriately attached to the verdict. Mehserle also got the least possible amount of jail time.

There are no "substantial legal issues" -- as the defense asserts -- for overturning Mehserle's involuntary manslaughter conviction. Mehserle was a trained police officer sworn to uphold the law. Acting under color of authority, he killed an unarmed man who did not pose a threat.

Are we to excuse the actions of police officers -- regardless of how egregious -- if they later say they made a mistake?

What kind of precedent would that set?

Mehserle, nor his supporters, appear to fully comprehend the gravity of his actions.

Mehserle told KTVU-TV Channel 2's Rita Williams that he never expected to go to jail. He thought he would be free to go home. Just like that.

Perry should deny Mehserle's request for bail. No reasonable court is going to overturn a conviction that is little more than a slap on the wrist.

Let Mehserle serve the remainder of his brief sentence and let Oakland finally begin to heal from this tragic affair.

Tammerlin Drummond is a columnist for Bay Area News Group. Contact her at tdrummond@bayareanewsgroup.com or Twitter.com/tammerlin.