Authorities trying to catch suspects buying sex with women or minors have long depended on Craigslist and Backpage, where an online personal ad could be viewed from anywhere in the world and elicit any number of responses at a given time.

But police and advocates across the state and nation are scrambling to adapt to new challenges after both sites’ personal ads were shuttered earlier this year, displacing sex trafficking onto mobile apps, obscure websites and foreign-owned sites that aren’t beholden to U.S. legal practices.

“It’s moving underground and on apps, so it’s becoming harder to find, to trace,” said LeVedra Vincent, an advocate at Mission 21, which works with youth in southern Minnesota to combat sex trafficking. “It’s becoming harder to identify and locate these victims.”

Authorities in Ramsey and Washington counties charged at least 29 men between late July and early August with a variety of crimes related to underage sex trafficking, from soliciting a child to prostitution. Most undercover stings were carried out in the days leading up to the Super Bowl, before Craigslist and Backpage ads went dark in March and April, respectively. Authorities leaned heavily on Craigslist, posting or responding to ads on the site in 15 of the cases that were charged. Backpage was used in six cases.

Authorities — a mix of the Minnesota Internet Crimes Against Children Task Force, the Minnesota Bureau of Criminal Apprehension (BCA) and various police and sheriffs’ offices — used other sites sparingly. The dating site Plenty of Fish and an app called Skout were each used to catch one suspect. The mobile app Grindr was used in three arrests, including that of St. Louis Park Rabbi Aryeh Leiv Cohen.

In three cases, authorities employed a website or app they declined to identify in charging documents.

Craigslist voluntarily removed its personal ads when the U.S. Senate passed a bill that would hold sites criminally and civilly liable for content published by users. The U.S. Department of Justice seized Backpage a month later as part of an investigation, calling the site “the Internet’s leading forum for prostitution ads.”

“We are currently looking through two dozen, maybe more, other websites this has moved to,” said Assistant Washington County Attorney Imran Ali, lead prosecutor of the East Metro Sex Trafficking Task Force. “Is it going to be more difficult for law enforcement and for prosecution on some of these cases? Undoubtedly it is.”

Some investigators predict a temporary drop in the number of cases that will be charged as law enforcement adapts to new trends.

“Going to multiple platforms can create difficulties for us,” said BCA Superintendent Drew Evans. “Part of it is just determining which ones to focus on at any given time and where it’s being utilized and where we need to go to target those people. People move around to different apps.”

Vincent said there are challenges to finding victims or suspects on mobile apps: Some are largely used to communicate with users within a certain distance from each other; many encourage one-on-one communication instead of mass responses; and apps don’t have traceable Internet Protocol addresses like a traditional online ad.

While well-known dating and social media apps and sites have absorbed some of the activity, Brandon Brugger, a Minneapolis police officer and human trafficking investigator, and Susan Webb, a civilian analyst with the department, said many new sites were created to fill the vacuum that was left behind.

“There are a ton of sites” that are new after the closing of Craigslist and Backpage ads, Webb said. “Daily we will find new sites.”

Investigators from multiple agencies declined to name specific sites and apps, saying it would harm their investigative advantage. There are also other concerns. Authorities had working relationships with Craigslist and Backpage, which occasionally removed suspicious ads themselves and responded to court-ordered subpoenas to turn over evidence or testify at trial.

“What we’re seeing now is that a lot of these websites are not in the United States,” Ali said. “We would no longer have that jurisdictional authority to request that subpoena. … We wouldn’t have the warrant ability to obtain certain information or even obtain banking documents.”

In Rochester, police have been pivoting away from traditional sites for years. Even while Craigslist and Backpage personal ads were active, investigators noticed that perpetrators were migrating to more exclusive sites. Rochester police have budgeted several hundred dollars each year for the past two years to pay for subscription-only websites.

“It used to be you could just pull up Backpage and it’s, ‘Oh, I know what we’re going to do today,’ ” said Rochester police investigator Capt. John Sherwin. “It requires a little more effort now.”

Authorities and advocates agree that closing Craigslist and Backpage ads was an important step in fighting sex trafficking. But advocates like Vincent, of Mission 21, and Pam Dixon, executive director of the World Privacy Forum, said it’s also cause for parents to educate themselves and their children about social media and mobile app safety.

“There’s always a new app that’s going to be popular with kids,” Dixon said. “Always.”