Attorney General William Barr said during a press conference on Monday that Apple had not helped the FBI crack into password-protected iPhones used by Mohammed Saeed Alshamrani, who is suspected of killing three people last month in a shooting at a Navy base in Pensacola, Florida.

The comments highlight law enforcement's frustration with encryption technologies that protect data so that neither Apple nor law enforcement can easily read it.

The speech also previews future clashes between technology companies and governments over whether to build "back doors" that would allow law enforcement elevated access to private data to solve crimes like terrorism.

"We have asked Apple for their help in unlocking the shooter's iPhones. So far Apple has not given us any substantive assistance," Barr said, next to a poster with a picture of the iPhones. "This situation perfectly illustrates why it is critical that investigators be able to get access to digital evidence once they have obtained a court order based on probable cause."

"We call on Apple and other technology companies to help us find a solution so that we can better protect the lives of Americans and prevent future attacks," he said. Barr has also clashed with Facebook over encrypted messages, which he called "data-in-motion" on Monday.

When Barr was asked if he was considering a court order to compel Apple to unlock the iPhones, he declined to comment. Last week, the FBI sent a letter to Apple's general counsel asking for help.

Apple did not respond to requests for comment. Last week, a spokesperson said: "We have the greatest respect for law enforcement and have always worked cooperatively to help in their investigations. When the FBI requested information from us relating to this case a month ago we gave them all of the data in our possession and we will continue to support them with the data we have available."

Apple was involved in a high-profile showdown with the FBI in 2016 when the Justice Department sued it to help it gain access to a phone used by Syed Farook, who was responsible for the mass shooting in San Bernardino, California, that left 14 people dead.

The FBI eventually said that it was able to gain access to the phone using a private vendor, which cracked the phone's security.

Apple has designed its devices with encryption, which means that it cannot access personal information on locked iPhones without the password to the phone. In the San Bernardino case, Apple said that in order to retrieve data that hasn't been uploaded to the company's servers, it would have to build special software.

Several vendors make devices marketed to law enforcement that can crack into iPhones, but the efficacy of those devices can increase or decrease based on security flaws Apple fixes.

Last week, an Apple employee focused on privacy who was speaking at a trade show defended Apple's use of encryption. She said the company has a team working around the clock to respond to requests from law enforcement but that she doesn't support building "back doors."