Police in Phoenix, Arizona, are investigating nine cases in the past 10 days where cars have been hit by bullets or other projectiles.

The shootings began on 29 August with two vehicles struck within half an hour and a third hit later that day. A fourth was hit two days later.

On Tuesday, an off-duty police officer was driving his own vehicle to work when the the passenger window was shattered. Another vehicle was hit a few minutes later.

A 13-year-old girl was injured by broken glass when a bullet shattered a windscreen in another incident.

Department of Public Safety Director Frank Milstead was not able to confirm whether the projectiles were all bullets, but added: "It is just a matter of time before there is tragedy."

Mr Milstead described the crimes as "domestic terrorism", reasoning that "anytime you have multiple shootings against American citizens on a highway, that's terrorism".

"They're trying to frighten or kill somebody," he said.

Local police, the FBI and the Bureau of Alcohol, Tobacco and Firearms are all working to find the culprit and undercover detectives, members of a SWAT team and a gang task force have also been brought in.

The police reward has been increased to $20,000.

Officials admit they do not know whether they are dealing with one culprit or whether a copycat is at work.

They have also not revealed whether the shooter seems to be working from inside a vehicle or alongside the road.

There have been other similar cases - including a man being convicted last year of terrorism charges after opening fire on a highway in Michigan because he thought drivers were part of a government conspiracy against him.

In 2003, a man in Ohio shot at cars and houses, killing one person before being caught and jailed.