Police chief Art Walgren knew how much the oil boom had changed this once-sleepy town when he spotted something that would have been unheard of not long ago: license plates from Sinaloa, Mexico, home to one of the world's most violent drug cartels.

Before, there was little chance police would see cars here from nearly 2,000 miles away. And little reason to worry about out-of-state plates. Now, though, police are scrambling to deal with new kinds of suspicious activity and threats that have cropped up along this frozen prairie.

The gusher of oil and money flowing from the Bakken fields has made policing more demanding and dangerous, forcing small-town officers, county sheriffs and federal agents to confront everything from bar fights to far-reaching methamphetamine and heroin networks and prostitution rings operating out of motels.

"It's not Mayberry anymore," says U.S. Attorney Tim Purdon. "Our police and prosecutors are going to have to adapt to keep pace. We have organized criminal gangs selling drugs, sex trafficking and out-of-state flim-flam men coming in. And the cases have become more and more complicated."

Most newcomers to the Bakken — which spans western North Dakota, eastern Montana and part of Canada — move here honestly in search of a new job or, in some cases, a new life. But more people also means more crime, overcrowded jails and overwhelmed police departments, often with relatively inexperienced officers racing from call to call.

"We are responding but we need to have more cops, more prosecutors and more judges," Purdon says. "We can't expect to move an incredibly large number of cases through the same machinery that's been in effect for the last 20 years."

Reinforcements are on the way. In November, the FBI announced it will open a permanent office in the Bakken — time and location to be determined — joining other federal and state law enforcement agencies helping local police crack down on newly emerging criminal enterprises.

"There used to be a saying that 40 below keeps out the riff-raff," says Steve Kukowski, Ward County sheriff. "That's not true anymore."

Not all crime is on the rise. In North Dakota, the number of murders dropped in 2013, but drug arrests increased nearly 20 percent compared with 2012. In Montana, oil patch arrests rose by 80 percent between 2008 and 2012, according to state Attorney General Tim Fox.

Here in Watford City, the police force has multiplied from four, including the chief, in 2010 to 19 sworn officers serving a population that could grow to 15,000 by 2017, a nearly tenfold increase since the last census. Even more dramatic: In 2006, there were just 41 calls for service, according to a 2013 North Dakota State University study. Last year, there were 7,414.

"It puts a lot of pressure on us," says Walgren, a 25-year police veteran who became chief last spring. "We're so used to trying to maintain that small-town attitude where you always wave at your neighbor and everybody's always your friend. Now, there are more people that you don't know than those that you do."

Watford City is a town in transition. Billboards beckon with job opportunities. Gas flares light the night skies, illuminating pump jacks that nod up and down, like giant birds feeding from the soil. Hulking rigs barrel down newly widened roads.

This is not the Wild West, as some media accounts have suggested, says Walgren, but police are navigating a new landscape: Embezzlements cases are larger. Officers now train motel workers on how to be alert for sex trafficking. And arrests have been made that have indirect ties to the Sinaloa cartel, the chief says.

What's happening here is not atypical.

In Dickinson, nearly 70 miles southeast, a highly visible four-person team patrols the bars on random Friday and Saturday nights to head off brawls that have become increasingly common.

"Some of the North Dakota niceness has left our community," says Dickinson Police Chief Dustin Dassinger.

Being a police officer has become more stressful, says Capt. Joe Cianni, a 21-year veteran. "This department wasn't used to dealing with major crimes involving weapons," he says. "In the past, it used to happen once every four to six months. Now it's once a week."

The hectic pace tends to burn out officers, and it's hard to recruit and retain new ones, Cianni adds, because of soaring housing costs and the remote location.

Stretched thin, Dickinson police don't have enough staff to conduct sex stings that result in misdemeanors. Some officers participated in one federal-state-local operation in late 2013 that had to be cut short when authorities ran out of jail space after arresting 11 men, says Purdon, the prosecutor. They'd answered a bogus ad police placed in Backpage.com purporting to offer sex with a 14-year-old girl.

"It shows a level of demand for sex with underage kids that's really scary," Purdon says.

State officials also are ramping up to pursue savvier criminals.

In Montana, local drug traffickers have been pushed aside by organized West Coast gangs and groups such as the Sinaloa cartel that "are pretty well-educated on law enforcement techniques," says Mark Long, the state Department of Justice's narcotics bureau chief. "It's 'been there, done that.' We have to get out ahead of the curve."

Larger communities also are encountering violent crimes.

In Williston, North Dakota, the epicenter of the oil boom, two men have been killed since 2013 outside two downtown strip clubs. The mayor recently lamented that Williston was devoting 80 percent of its late-night policing to the clubs, taking away needed resources from other areas.

In Ward County, two hours east of Williston, the sheriff is facing a problem that exists in many parts of the oil patch: an overcrowded jail. The county is now transferring inmates 110 miles away to deal with the problem. It has also stopped jailing people charged with low-level crimes, instead requiring them to sign a pledge to appear in court.

Next month, voters will be asked to approve up to $41 million in borrowing to renovate the courthouse and expand the jail by 100 cells.

"It's extremely frustrating," Kukowski says. "I never believed we would be facing some of the issues we're facing now.... We just have to deal with them."

___

Sharon Cohen, a Chicago-based national writer, can be reached at scohen@ap.org.