It's not clear why a New York police officer shot and killed an unarmed Army reservist on the Grand Central Parkway last week but Police Commissioner Ray Kelly says a grand jury investigation would be the best way to find out.

Kelly said the department had spoken to a witness, but not the police officer who shot 22-year-old Noel Polanco early Thursday morning.

“That is out normal protocol. The reason we don’t do that is because there’s the potential of giving immunity to the shooter in cases like that,” Kelly explained. He did say other police officers on scene had been interviewed.

The commissioner said the police investigation is still going forward. Queens District Attorney Richard Brown is also investigating the events of that morning, when NYPD Detective Hassan Hamdy, a 14-year veteran of the force, pulled Polanco over on the Grand Central Parkway last.

A passenger in the car said Polanco did nothing to provoke police. An off-duty police officer was asleep in the back of the car.

Meanwhile, the City Council is expected to hold a hearing Wednesday on a package of bills that would create an Inspector General’s office to monitor the NYPD.

Mayor Michael Bloomberg said that would politicize control of the police department.

“If people aren’t happy with police department and what Commissioner Kelly has created, and all of the 55,000 people that work in the PD, I don’t know what would make you happy,” Bloomberg said. “I think this police department is well supervised, it’s well lead, and the last thing we need is to have some politician or judge getting involved.”

Bloomberg said the statistics show the murder rate continues to fall, as do the number of shootings in the city.