Is sex trafficking enough reason for sneak-and-peek cameras? Legal experts aren’t so sure.

The cameras were secretly recording New England Patriots owner Robert Kraft both times the billionaire showed up at a day spa to solicit sex, police say.

Investigators said they surreptitiously watched Kraft and others last month, thanks to the handful of cameras they hid inside the Orchids of Asia Day Spa, a massage parlor in Jupiter.

After the 9/11 attacks, the U.S. expanded the use of secret surveillance in the war on terror. But the tactic has spilled over to infiltrating other crimes, most recently the alleged sex acts at the Jupiter business.

In such cases, so-called sneak-and-peek warrants let authorities access private property so the government can secretly do a search without notifying people under investigation.

But legal experts say the police likely have taken an unprecedented step by hiding cameras on private property for an investigation into prostitution and human trafficking. Celeste Higgins, a University of Miami law professor, said she never saw a sneak-and-peek warrant during her 25 years as a federal public defender. “It’s a very rare thing to have,” she said.

And when these types of searches “get used for a common crime — quite frankly, as insignificant as basic prostitution — it’s very, very troubling,” she said. “That means there is no limit when they could use” it.

“Putting devices into locations is really the ultimate invasion,” she said. “How is this a terrorism case?”

Police justify their use of the cameras because they sought to crack down not only on prostitution, but the egregious crime of human trafficking. And they said the cameras helped them ensnare two dozen men on charges of soliciting sex. Perhaps the highest-profile of those charged is Kraft, 77, who this week pleaded not guilty to soliciting prostitution.