Chicago - Prosecutors moved quickly to charge a white Tulsa, Oklahoma, police officer with manslaughter in the fatal shooting of an unarmed black man whose vehicle broke down in the middle of a street.Now, they have to prove it.The Tulsa County District Attorney's Office offered just a glimpse of the existing evidence when it charged Officer Betty Shelby with first-degree manslaughter in the death of Terence Crutcher.A video from a police helicopter and another from a dashboard camera show the incident, but they are partly obscured and neither shows unambiguously that Shelby had acted criminally.Crutcher can be seen walking with his hands in the air toward his SUV, stopping by the driver's window and then falling as a shot is fired.Based on what's been made public thus far, some legal experts say prosecutors may have the tougher task of proving the officer's guilt beyond a reasonable doubt at a trial."I would much rather defend this case than prosecute it," said Lee F Berlin, a former assistant district attorney-turned-defence lawyer in Tulsa.Here's a look at potential strengths and weaknesses of prosecutors' case:What's required to prove the charge?Answer: Prosecutors aren't saying they think Shelby planned to kill Crutcher on the evening of September 16.If they believed there was premeditation, she could have faced a murder charge. What they would have to prove to jurors is that Shelby fired the fatal shot by succumbing to an irrational fear that the 40-year-old Crutcher was about to harm or kill her.If convicted, Shelby would face anywhere from four years to life in prison.Does the case hinge on one question?A: If so, it's this, said former federal prosecutor Jeff Cramer: "We know she was afraid. But was that fear a reasonable fear? Everything rises or falls on the answer."Fear of a rhino charging at you might well be reasonable; a fear of an ant crossing your path almost certainly isn't. Assessing reasonable fear in police shootings is rarely straightforward.A 1989 US Supreme Court ruling provided some guidance. Noting that officers "are often forced to make split-second judgments", it found that the reasonableness of an officer's use of force should be judged "from the perspective of a reasonable officer on the scene, rather than with the 20/20 vision of hindsight".At any trial, prosecutors are likely to call a lot of witnesses, including police officers, to try and show that other officers in Shelby's position wouldn't have acted as she did.Cramer said the defence will almost certainly have to call one particular witness to counter that testimony: Shelby herself."She has to take the stand," said Cramer, now a Chicago-based managing director of investigations at Berkeley Research Group. "She is the only one that can testify to what she was thinking."What might support the charge?A: A two-page affidavit filed with the charging document offers a concise account of how events unfolded but only hints at what prosecutors could argue at trial.It says, for instance, that Shelby had already checked the driver's side of Crutcher's SUV and saw no weapons minutes before she opened fire.That would seem to cast doubt on statements that she perceived an imminent threat and fired because Crutcher started reaching through the driver's side window and she thought he might be going for a weapon.The affidavit also notes that a fellow officer next to Shelby pulled out and fired his stun gun, not his firearm. His decision to use a stun gun would underline the point that he calculated the situation wasn't as ominous as Shelby described and that it called for the use of less-than-lethal force.What are the possible weaknesses?A: Parts of state's affidavit appear to support Shelby's contention that Crutcher's behaviour could be seen as threatening, said Berlin.Prosecutors, themselves, seem to accept that Crutcher wasn't complying with Shelby's orders for him to stop walking away toward his SUV.The defence could try to argue that, among other things, the mere fact that Crutcher wasn't complying with some of Shelby's orders while she had a gun pointed at him itself legitimises Shelby's fears that he might have been under the influence of drugs, was erratic and a potential threat.Could the charge change?A: Prosecutors often start by charging the most serious crime that they can conceivably prove, which can give them leverage in trying to get a suspect to plead guilty to a lesser charge.As more evidence emerges, the district attorney's office could also reduce the charge from first-degree to second-degree manslaughter, which requires proof of extreme negligence.A judge could also decide that the evidence doesn't support first-degree manslaughter but does justify a trial on second-degree manslaughter, which carries a maximum sentence of just four years behind bars.