Cameras on Portland police cars are starting to collect information on local vehicle license plates, and can track up to 128,000 vehicle license plates per day.
That means that if authorities are investigating a murder and want to check on the whereabouts of a suspect's car three years ago, they can tap into a computer and perhaps find out where it was on a particular day.
The system is already solving dozens of crimes. But some civil liberties advocates have reservations.
Officer Garrett Dow said the value of the system was evident to him as soon as he got it.
"We turned the system on and within five minutes I was able to locate a vehicle involved in an assault," he said.
These days, whenever he climbs into his blue and white squad car, he switches on his Automatic License Plate Reader. Four cameras mounted on the roof take pictures of all the cars he passes _ whether parked or driving. They then feed license plate numbers into a computer. A dash-mounted screen pings each time the computer logs another plate's number.
Most of the time, the system just runs in the background, filling up a hard drive. But if the system sees a number plate that's been flagged, whether because the car's been stolen or it belongs to a wanted felon, Officer Dow is alerted.
Lieutenant John Scruggs, who oversees the system for Portland Police, said the bureau recovered more than two million dollars worth of stolen cars last year.
"We're not necessarily recovering more cars because of it, though it seems like we're getting more people in custody, because we're catching the car while it's being driven, versus being parked," Scruggs said.
He said the bureau now has cameras mounted on 16 police cars that have helped solve crimes ranging from hit and run to fraud,
"We had one particular case, where we had an identity theft suspect, 100 counts of identity theft," Scruggs said. "We couldn't locate him because he's living out of different people's houses. He's living on the street in his car. The detective had no ability to follow up. But they put the plate in our system and located the vehicle, literally three days within putting it in there."
But some are concerned that the police could use this powerful system to find out, for example, whether a local politician is having an affair; or to learn the identities of people getting together for a political rally.
David Fidanque, the executive director of the American Civil Liberties Union of Oregon, doesn't mind the system being used to catch criminals. But, he says, the government shouldn't be collecting massive amounts of information on innocent people, "What the ACLU wants to do is not to prohibit the police from using this technology," he said, "but put strict limits on how long the data can be retained if there isn't a hit on the license plate."
Fidanque says the ACLU is drawing up legislation that would require police to wipe the record of any number plate logged after 24 hours -- unless it popped up because of a criminal issue.
"The underpinning of our constitution is that we have the right to be left alone by government, unless the government has some specific evidence that we've done something wrong," said Fidanque.
"And then the government can search our person, our property, if the government has probable cause, and they have approval from a judge," Fidanque said. "So we're saying, let's just stick by those rules. Those rules aren't an obstacle to good law enforcement, they're a road map to good law enforcement."
Lieutenant John Scruggs is well aware of the ACLU's objections. But he says after talking to law enforcement agencies across the country, he's set Portland's system to clear number plates after four years.
"Realistically the statute of limitations runs out on a lot of major felonies at seven years," he said. "And after four years, if we haven't solved a crime the likelihood is pretty low. And in talking with an agency down in California, they discussed a case where they had a plate read that was valuable four years after the crime. And so I settled on four years."
And Scruggs says the Bureau changed its policy after talking to the ACLU about collecting plates at public gatherings. Officers are no longer supposed to take the cars to rallies or churches or other gathering places, unless they are out on a call.
Scruggs also said the Bureau's policy is that officers have to log onto the system to search it, and they can only use it if they're working on a criminal investigation.
"If they log on and they don't have a relevant case that they're looking into, they're violating our policy," said Scruggs.
Earlier this year, the ACLU successfully backed an effort prohibiting law enforcement from using aerial drones for surveillance without a court order.
But they can be used in an emergency situation -- like a search and rescue, or the pursuit of a fleeing suspect.
A couple of other details: the pictures taken by the license plate system aren't usually good enough to show who's driving; and the system is only about 80 percent accurate. That's because sometimes there's mud on a number plate or a trailer hitch in the way.