By MICHAEL TARM, AP Legal Affairs Writer

CHICAGO (AP) — A federal judge in Chicago is the latest to weigh in on still-disputed legal questions about whether law enforcement can order suspects to put their fingers on fingerprint sensors to unlock a smartphone, concluding it's not necessarily a violation of protections against self-incrimination.

U.S. District Judge Edmond Chang's recent decision overturned a federal magistrate's ruling in a child pornography investigation that said forcing four residents of a home to each place their fingers on Apple iPhone Touch ID sensors would violate those Fifth Amendment rights.

Courts have long held authorities can't make suspects reveal combinations to a safe because doing so amounts to being forced to testify against themselves. But judges have simultaneously maintained that authorities can demand the safe key because it's an object and turning it over doesn't depend on verbal expressions.

Wider use of fingerprint scanners has complicated how such guidelines are applied, with courts across the country applying them differently. The Supreme Court hasn't yet taken up the specific issue.

The issue arose in Chicago when the FBI early this year sought a wide-ranging search warrant in the child pornography case that included a request for permission to require that the two parents and two adult children who lived at the home unlock any iPhones or iPads in the house that used fingerprint sensors.

In her February ruling, U.S. Magistrate Judge Sheila Finnegan broadly OK'd the warrant. But she denied the permission on fingerprints, saying whoever's finger unlocked the phone could be seen as admitting responsibility for any illegal images on the device, which effectively would be self-incrimination.

The U.S. attorney's office in Chicago disagreed with that ruling and sought the review by Chang. Prosecutors argued in a March filing that the request to force the family to put their fingers on the iPhone sensor wouldn't be a form of expression, but would be the same as asking suspects to hand over keys to a safe.

"Touch ID and similar technologies have made fingerprints akin to a key," the filing said. "Instead of carrying that key on a metal ring, Apple users now carry five potential keys on each hand."

The federal defenders' office in Chicago also filed a written document in March backing the magistrate's conclusions and arguing that the vast personal information on a smartphone meant a fingerprint sensor isn't comparable to key. "This present case," it said, "falls outside of this dated analogy."

But Chang said the analogy was a good one, writing in his opinion that "compelling physical access to information via the fingerprint seizure is no different from requiring someone to surrender a key to a safe whose contents otherwise would not be accessible to the government."

Chang decided the issue last month, though his written ruling wasn't unsealed until this month. Many of the filings in the case, including ones with information about the identities of any suspects and details of the allegations, remain sealed.