WATERVILLE — Ten car thefts in six weeks, sometimes two in one day.

Waterville police have warned people to stop leaving keys in their cars as they investigate a string of daytime vehicle thefts from parking lots.

In each case, someone has climbed into an unlocked car or truck with the keys still dangling from the dashboard and then driven away. And in each case, the police have found the car a few blocks away, usually in another parking lot.

Investigators have found fingerprints, but so far they have no suspects.

“I suspect, initially, it was a crime of opportunity, and it just kept going from there,” Deputy Police Chief Joseph Massey said Wednesday.

“They got away with it once and said, `Why walk, when you can ride?”‘ he said.

The string of thefts began in December in the Concourse, a large parking area near the city’s downtown.

People would report cars and trucks missing when they returned from shopping, and the police would find them a few hours later, often just a few miles away.

The cold weather has probably hastened the crimes, since people are more likely this time of year to leave their vehicles idling, Massey said.

The most recent thefts in the city happened Tuesday evening.

At 4:45 p.m., someone reported a Honda Civic stolen from outside the Waterville House of Pancakes, off Kennedy Memorial Drive. Eighteen minutes later, police received a report of a Toyota truck missing from outside the Elm Plaza, on the other side the city.

As police officers met with the owner of the stolen Toyota, they discovered the stolen Honda parked nearby.

Later, the police found the stolen truck outside the JFK Mall, which is near the pancake restaurant.

“They’re leaving them where they can be easily found,” Massey said. “We’re pretty sure it’s the same people.”

The thieves don’t always stop in Waterville. Sometimes, they like to cross the city line.

Police in neighboring Fairfield joined the investigation after at least two stolen vehicles were found near the town’s Main Street, one behind a Rite Aid, another in a municipal parking lot a stone’s throw from the police station.

About the same time, two or three cars reported missing from Fairfield were recovered in Waterville, Fairfield police Capt. John Emery said Wednesday.

“Are we looking at a connection?” he asked. “Yes, we are.”

The most recent thefts in Fairfield happened early Friday morning when someone on the Middle Road reported his Chevrolet pickup truck missing.

As an officer spoke with the owner, the stolen truck passed by. A Fairfield police officer gave chase and recovered the truck in Skowhegan, after it ran over a spike mat placed across the road by police in that town.

A Fairfield man, Douglas J. Chartrand, 32, was arrested on charges that included theft and drunken driving.

Fairfield police also consider him a suspect in another vehicle theft the same night, involving a Ford Ranger truck that had been stolen in the Fairfield village of Shawmut and recovered a few miles away.

“We were kind of hoping that, after the other night’s arrest, some of this would slow down,” said Emery.

In Waterville, Massey said he hopes people heed the warning and remove the keys from their cars. Otherwise, he’s afraid a simple car theft might become more violent.

“I’m concerned that an owner will come out while a crime is in progress,” Massey said.