The FBI is pressing Apple for help opening iPhones that belonged to the Saudi military student who killed three people last month at a naval base in Pensacola, Fla., signaling a potential revival of the fight between the federal government and Silicon Valley over encryption technology.

On Monday, the FBI’s general counsel Dana Boente wrote a letter to Apple’s top lawyer, Katherine Adams, seeking the tech giant’s assistance.

“Even though the shooter is dead, the FBI, out of an abundance of caution, has secured court authorization to search the contents of the phones in order to exhaust all leads in this high priority national security investigation,” Boente wrote.

“Unfortunately, FBI has been unable to access the contents of the phones,” the letter said, even after asking private technology experts if they could help agents crack them. “None of those reachouts has shown us a path forward.”

The letter was first reported by NBC News.

In a statement, Apple said it had already helped FBI agents on the Pensacola case by sharing relevant data in its cloud storage. Apple and other companies have said that encryption on phones is an important safeguard protecting millions of consumers against hackers and other criminals.

The circumstances of the request — a dead killer’s phone which FBI experts say they cannot crack — is reminiscent of the high-stakes legal standoff four years ago over the iPhone of a San Bernardino, Calif., county employee who, along with his wife, killed 14 people in a shooting rampage.

That legal battle between the Justice Department and Apple sparked a national debate about the competing interests of national security, law enforcement, personal privacy and giant tech firms. But the larger legal question of whether the government could force companies to provide access to electronic devices was never answered by the courts because, amid their fight, the FBI found a private firm that could access the gunman’s phone.

Since that case, others have popped up in which the FBI could have gone to court to try to force Apple’s hand, including with the phone of a Texas man who killed 26 people in 2017. But none of those investigations became a test case on encryption.

Attorney General William P. Barr and other law enforcement officials have said that widespread encryption on consumer devices makes those devices effectively off limits to investigators even with a court order, hampering investigations into terrorism, child abuse and other crimes.

“We have the greatest respect for law enforcement and have always worked cooperatively to help in their investigations,” Apple’s statement says. “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.”

Boente’s letter notes that the phones at issue in the Pensacola case are older models, one an iPhone 7 and the other an iPhone 5. That would suggest at least the possibility that software engineers would be able to find a way to access the devices’ data without the passcode, because the technology used in older devices is sometimes easier to crack.

There is another complicating factor to the request — one of the phones was struck by a bullet, according to people familiar with the case. It is unclear how significant that damage is in regard to accessing the data, but investigators are particularly interested in accessing that device in case he shot it deliberately to keep investigators from seeing the information it contains, these people said.

The Pensacola gunman, a Royal Saudi Air Force member named Ahmed Mohammed al-Shamrani, was training at the base when he used a legally purchased 9mm handgun to go on a rampage, authorities have said. He was shot dead by a sheriff’s deputy.

Shamrani left hints that he was motivated at least in part by his hatred of American foreign policy and military might. Investigators said they believe Shamrani was the author of an anti-American screed posted on Twitter shortly before the shooting, according to a law enforcement official. The Saudi government has pledged to cooperate with the FBI investigation.

In the San Bernardino case, which appeared headed for a precedent-setting Supreme Court showdown between Silicon Valley and the federal government, the FBI went to court seeking a judge’s order to compel Apple to help them open the phone.

Then, as the case was working its way through the legal system, the FBI announced that an outside firm had found a solution. That unexpected development solved the FBI’s immediate problem, but it set back the Justice Department’s efforts to pressure tech firms into providing a means of access to suspects’ phones. A Justice Department inspector general review of the FBI’s handling of the San Bernardino case later found that key elements of the FBI’s technology division were not communicating well internally to try every possible way to open the phone.

James Baker, who served as the FBI’s general counsel during the encryption battle over the San Bernardino phone, said there are definite similarities to the new case, even though he now has a more favorable view of encryption than he did then.

“It strikes me that the government today is in a similar position that we found ourselves in, in terms of owing a duty to the victims and their families to try to pursue every logical lead,” said Baker, now the director of national security and cyber security at the think tank R Street Institute.

When it comes to the debates over encryption and constitutional protections against unreasonable searches, Baker said, “The problem is not the Fourth Amendment, the problem is our U.S. statutes. No court has found that they empower the government to do what the government wants them to do.”