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With fingerprint sensors, it's easier than ever to unlock your phone — especially for the government.

In February, a woman in Glendale, California, was compelled by a search warrant to unlock her iPhone with her finger, an action that some experts say falls in a legal gray zone.

That happened around the same time as the public fight between Apple and the FBI over unlocking an encrypted phone used by one of the shooters in the San Bernardino massacre. And legal questions could arise over the use of the Textalyzer, which lets police know whether or not a phone was used to text while driving.

"Technology is always moving faster than the law," Valerie Barreiro, director of the University of Southern California's Intellectual Property and Technology Law Clinic, told NBC News.

"At some point, the courts will be forced to catch up," she said. "Right now, businesses want to know what the rules are."

Read More: FBI Accessed iPhone in Terror Case, Raising More Questions About Key Issues

Apple, Samsung and other phone manufacturers have been pushing fingerprint sensors as a convenient security measure. Legal complications, however, might force them to reconsider their marketing strategy.

What's in a fingerprint?

It's not clear why the FBI wanted the iPhone of Paytsar Bkhchadzhyan, 29, who was arrested on charges related to identity theft. It was found in the Glendale home of Bkhchadzhyan's boyfriend, a suspected gang member, the Los Angeles Times reported in April.

The search warrant did not specify the reason the FBI wanted access to the phone, only that it was granted. It's a problematic warrant for several reasons, according to Barreiro and Neil Richards, a privacy law professor at Washington University.

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Opening the phone with a passcode would have violated Bkhchadzhyan's Fifth Amendment protection against self-incrimination, Richards said. But the use of a fingerprint provides law enforcement some legal cover.

"Most people don't draw a distinction between a fingerprint and a password, but the law does," Richards told NBC News.

Typing 1-2-3-4 to access a phone counts as testimonial, in that it's information that can be used to authenticate the contents of a device. Touch ID, the fingerprint sensor that became available with the introduction of the iPhone 5S in 2013, does pretty much the same thing, except with biometric data instead of a password.

Read More: Leak of Senate Encryption Bill Prompts Swift Backlash

Yet, laws written before smartphones were invented treat them differently. Law enforcement is allowed to collect physical evidence during the course of an arrest, such as DNA evidence or fingerprints. The issue, Barreiro said, is that the FBI didn't really treat Bkhchadzhyan's fingerprint like physical evidence.

"It's not the same as DNA, where you’re trying to establish whether or not someone is at the scene of the crime," Barreiro told NBC News. "That fingerprint is opening up a window into your entire life."

The Supreme Court agrees. In the 2014 case Riley v. California, it held that law enforcement needs a search warrant to open a phone that agents confiscated during an arrest.

"Modern cell phones are not just another technological convenience," wrote Chief Justice John Roberts in the majority opinion.

"The sum of an individual’s private life can be reconstructed through a thousand photographs labeled with dates, locations, and descriptions," he wrote, adding that "the same cannot be said of a photograph or two of loved ones tucked into a wallet."

The invention of the cloud, Roberts wrote, only increases the amount of information about a person that can be learned from having access to their smartphone.

In New York, the state legislature is considering a roadside test called the Textalyzer. It would let police officers plug a cellphone into a laptop and determine if it was used while driving.

Every day in the United States, eight people are killed and 1,161 are injured as a result of distracted driving, according to the CDC. In that context, it makes sense to have what is essentially a Breathalyzer test for texting while driving.

But if officers look at the content of a phone, as opposed to solely whether or not it was in use, the Textalyzer could present a host of privacy problems, Barreiro said.

Read More: Facebook's WhatsApp Strengthens Encryption for 1 Billion Users

There is no easy answer to these questions, she added. It will probably take something similar to Bkhchadzhyan's case getting appealed and then heading to the Supreme Court to get any kind of clear answer on what law enforcement can and can't do with a confiscated phone.

Until then, tech companies might want to rethink how they push features like Touch ID. When a court ordered Apple to unlock the iPhone used by the San Bernardino shooter, the company didn't mince words, calling it a "dangerous precedent" that would "hack our own users and undermine decades of security advancements."

If more law enforcement agencies force people to unlock their iPhones, tech companies could urge their customers to take privacy precautions.

"They’re going to start thinking twice about nudging people toward just using fingerprints," Richards said. "It is secure against private parties, but under current law, it’s not as secure against the government."