A magistrate ruled police could force a man to unlock his phone - is that legal?

When Fairfax County, Virginia, police arrested a man last month in connection with the brutal sexual assault in May of a 15-year-old with developmental delays, they seized a piece of evidence that might hold crucial evidence of the crime: his iPhone.

The phone's GPS might have placed Kevin Caldwell at the scene, contained texts between him and his alleged accomplice or even video of an incident, but authorities quickly realized they could not access any of it.

The phone required Caldwell's fingerprint to unlock it.

A detective resorted to a novel and controversial approach: She went to a magistrate and got an order to have Caldwell provide his fingerprints to gain access, but police said they ultimately decided not to follow through with the thumbprint.

In an age of increasing encryption, law enforcement officials say compelling a person to cooperate is sometimes the only way to retrieve make-or-break evidence. They have asked magistrates and judges to order suspects to give up their fingerprints to unlock phones, as well as face scans and passwords.

The trend has touched off heated legal battles in a number of state and federal courts over the constitutionality of such searches, resulting in a welter of legal opinions. Defendants often argue that such searches violate their Fifth Amendment rights against self-incrimination.

"These are newer technologies for securing devices, but they've been around for several years now," said Andrew Crocker, a senior staff attorney for the Electronic Frontier Foundation. "You've started to see police trying to force people to unlock their phones with their fingerprints or faces and courts having to deal with that."

Fairfax County police said the incident involving the 15-year-old girl occurred May 16. The girl told detectives she went to a family event at the Mount Vernon Country Club, where she got into a disagreement with her brother, according to a search warrant filed in Fairfax County.

The teen went out to get some air and ended up walking into a Wingstop restaurant on Cooper Road, according to the search warrant, where she asked Caldwell for a ride, saying she was lost. He declined, the search warrant states.

Caldwell and a second man, Andrew Collins, who was an employee at the restaurant, followed the teen into the bathroom and blocked the door, she told detectives, according to allegations in the warrant. The girl was taken to another location she described as "sticky," where both men allegedly assaulted her, according to the search warrant.

After an investigation, Caldwell, 21, and Collins, 22, were arrested in July. Caldwell was charged with forcible sodomy, and Collins was charged with animate object penetration. Neither has a fixed address, and they have yet to enter pleas. Caldwell's attorney declined to comment, and it could not be determined whether Collins had an attorney.

Fairfax County Police Detective Alyson Russo applied for a search warrant for Caldwell's fingerprints to unlock the phone in late July, citing a 2014 Virginia Beach Circuit Court ruling that touches on many of the crucial legal arguments surrounding such searches.

In that case, police wanted the state to force a man charged with assault to give up his passcode or a fingerprint to unlock a phone they suspected contained video evidence of the assault. The defendant objected, saying it would violate his Fifth Amendment protections against providing incriminating testimony against himself.

Crocker said the courts generally have held that the Fifth Amendment bars law enforcement from forcing people to provide testimony that would reveal the contents of their minds.

The judge ruled that the state could compel the man to produce a fingerprint, but not a passcode.

"The fingerprint, like a key ... does not require the witness to divulge anything through his mental processes," the judge wrote in his opinion. "Unlike the production of physical characteristic evidence, such as a fingerprint, the production of a password forces a 'defendant' to disclose the 'contents of his own mind.' "

Other courts have come down differently. Some have compelled defendants to give up passcodes for phones, even jailing some who have refused. In recent years, courts in California, Illinois and Idaho have found that compelling the production of fingerprints is unconstitutional under the Fifth Amendment, although most of those decisions have been reversed.

The Fairfax County public defender's office, which is representing Caldwell, objected to the police's quest for his fingerprints. "It is a violation of the 5th amendment to compel someone's fingerprint," Dawn Butorac, a public defender, said in an email.

But Fairfax County Police Capt. Eli Cory said the searches were lawful. He said that such searches are rare and that the department did not maintain statistics on how often they happen. Because the case is pending, he declined to say whether police ever gained access to Caldwell's phone through means other than the fingerprint.

"We face encryption all the time in some shape or form," Cory said. "It's something we have to overcome in the context of law."

In major rulings in recent years, the Supreme Court has strengthened privacy protections around police searches of cellphones, citing the wealth of information they contain. Last year, the high court ruled that police generally need to obtain a warrant to get cellphone location data; and in 2014, the Supreme Court ruled that authorities must obtain a search warrant to search the contents of a cellphone.

Riana Pfefferkorn, associate director of surveillance and cybersecurity at the Stanford Center for Internet and Society, said the issue of compelling defendants to give up access to their phones may be an issue the Supreme Court ultimately takes up, too.

"The constitutional questions are still developing," Pfefferkorn said.