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These impacts include sugar-beet farmers on tractors competing for space on tight two-lane highways with rumbling rigs that rush sand, water and heavy machinery to drill sites. Drunken driving arrests are way up, and police report seizures of uncommon illicit drugs.

"Heroin is starting to come back. The drug activity has really changed in this region," said Doug Colombik, the Miles City police chief.

Cops on the beat feel a difference, too.

"The level of aggression that we're met with when we're responding (to a call) has really increased," Mark Kraft, 33, a night officer for the Sidney Police Department, said during ride-along with a McClatchy reporter. "It makes our job a little more dangerous than it was a couple of years ago."

The big wakeup call came in early January, when schoolteacher Sherry Arnold went for a morning jog in Sidney and never returned.

Her remains were found months later across the state line near Williston. Police said the 43-year-old cancer survivor was kidnapped and killed. Two Colorado men who came to the area in search of work in the oilfields are charged in her death.

Arnold's slaying brought soul-searching over the costs of a transformative oil boom.

Almost to a person, everyone interviewed in the region complained they no longer recognize people in the grocery store, and that they now must lock their doors. A large town here is home to fewer than 6,000 people, and leaving doors unlocked and keys in the car is the very definition of small-town life.

"I think whenever you don't know people, you become suspicious of them. You just have to remember that not all strangers are bad," said Maj. Robert Burnison, Sidney's assistant police chief. "I tell people that, and to be aware of their surroundings ... just be cautious. You don't have to be afraid."

Burnison recently counted 17 out-of-state license plates in the local grocer's parking lot. This flood of new American workers, dubbed "patch rats" by locals, is also clogging up the criminal justice system in eastern Montana, he and others said.

"We average about a DUI a day now," said Judge Gregory P. Mohr, a city judge in Sidney, whose office is strained by mounting demands and no comparable increase in revenues. "All of the oil money goes west (to the state capital), but we need it here."

Montana has a "three strikes" policy, meaning a fourth drunken driving arrest is treated as a felony charge and conviction results in a 13-month prison sentence. Many of the arrested have strikes in other states, and Montana's policy counts them.

"We're up in felonies all over the place here. What we're seeing is a lot of out-of-state (resident) felonies. These are extraordinarily taxing to our system," said Sheila M. Newman, the deputy public defender in Miles City, who now spends much of her time researching the laws of other states. "I'd like to have another full-time employee, but the legislature says the public defender system is maxed out."

While most oil workers have clean records, some arrive with outstanding arrest warrants. And others are criminals taking advantage of laws that allow extradition only from contiguous states. Authorities in Montana get stuck with criminals who have arrest warrants but cannot be extradited.

It all explains concern that eastern Montana will inevitably resemble Williston, which looks like a chaotic oilfield staging center, and earlier this year was named by the Census Bureau as the fastest-growing micro area of the nation between April 2010 and July 2011. Roadside businesses tied to the industry dot both sides of the highway for seven miles out of town. Williston is home to exploding crime, and police think that the Hells Angels motorcycle gang now distributes drugs in the region.