SIOUX FALLS, S.D. — Sioux Falls has become a popular stopping-off point for sex traffickers heading to the booming western North Dakota oil patch, where pimping and prostitution are on the rise, authorities say.

Police statistics show that about two-thirds of the 78 people who have been arrested in the past two years in connection with prostitution in Sioux Falls, South Dakota's largest city, came from Wisconsin, Minnesota and other states. Authorities don't see any signs of the activity slowing, Milwaukee FBI Agent Chad Elgersma said.

"It's logical for traffickers traveling to North Dakota for the oil boom to use it as sort of an in-between stop, as another area to make money on their way up," Elgersma told the Argus Leader newspaper ( ).

In North Dakota, the U.S. attorney, the state attorney general and others have sounded alarms about a surge of trafficking linked to the state's oil boom and its increasing population of transient workers. The Legislature is considering several measures aimed at combating the increasing incidence of prostitution in the state, including increasing the penalties for sex traffickers.

The number of pimping and prostitution arrests in Sioux Falls began to increase in 2011, when North Dakota oil production began to rapidly climb. Sioux Falls police that year shifted responsibility for investigations to the department's street crimes unit and shifted the focus to arresting pimps rather than the prostitutes, who often are victims, spokesman Sam Clemens said.

"When street crimes took over those investigations, they started making them (the pimps) more of a priority," he said.

The entire community suffers if Sioux Falls becomes a known haven for sex traffickers, said Audrey Willard, a member of the Junior League of Sioux Falls and a committee member of its human trafficking awareness board.

"Hopefully we can send a message to traffickers that we won't tolerate it here," she said.