From the Wall Street Journal By Devlin Barrett

Updated March 28, 2016 10:20 p.m. ET

WASHINGTON—The government said Monday it had cracked a terrorist’s iPhone without Apple Inc.AAPL 0.28 % ’s help and is seeking to drop its legal case to force the tech giant to unlock the device.

The move was announced in court papers filed Monday in a dispute over a phone seized in the investigation of a Dec. 2 terror attack in San Bernardino, Calif. The filing signals a temporary reprieve from a high-stakes fight between Washington and Silicon Valley over privacy and security in the digital age. For now, the Federal Bureau of Investigation is focused on reviewing the information contained on the phone, which was unlocked with help from a third party the government has refused to identify.

The filing doesn’t indicate what method the FBI used to access the data on the phone, nor does it say what, if any, evidence related to the attack was found on it. A government official said the method to unlock the phone wasn’t developed by a government agency, but by a private entity. Officials declined to say whether the same method could be used to open other versions of the iPhone using other operating systems.

The filing short-circuits a pending legal showdown over whether the government can force technology companies to write software to aid in criminal investigations, but it is unlikely to avert the long-term conflict over how secure electronic communications should be, and what firms should have to do to help the government access their customers’ data.

The decision by federal officials to drop the case comes a week after prosecutors bowed out of a planned courtroom battle, telling the magistrate judge in the case that they may have found a new way to access the phone without Apple’s help.

In Monday’s filing, prosecutors revealed the method had worked and Apple’s assistance was no longer necessary.

Justice Department spokeswoman Melanie Newman said the FBI “is currently reviewing the information on the phone, consistent with standard investigatory procedures.”

The Justice spokeswoman also signaled that while this particular phone is no longer at issue, the broader fight over encryption-protected technology is likely to continue.

“It remains a priority for the government to ensure that law enforcement can obtain crucial digital information to protect national security and public safety, either with cooperation from relevant parties, or through the court system when cooperation fails,” she said.

Late Monday, Apple said the case should never have been brought because it was wrong and would have set a dangerous precedent. The company also said it would continue to help law enforcement with investigations, while continuing to improve the security of its products.

“Apple believes deeply that people in the United States and around the world deserve data protection, security and privacy. Sacrificing one for the other only puts people and countries at greater risk,” the company said. It didn’t say whether it would seek information about how the FBI unlocked the phone. An Apple spokeswoman declined to elaborate.

The Justice Department had sought Apple’s help to gain access to a phone used by one of the shooters in the San Bernardino, Calif., terror attack. Photo: Mark Lennihan/Associated Press

Since the company’s fight with the government became public last month, Chief Executive Tim Cook has staunchly defended what he says are critical privacy interests at stake.

Since 2014, Apple has said that it encrypted data on iPhone software and protected it in a way that gave the company no way of removing information from locked devices. But the government’s announcement that it now has a way into gunman Syed Rizwan Farook’s iPhone suggests that there is at least one previously unknown security flaw in at least one version of the phones.

Morgan Reed, executive director of ACT, the App Association, called the move a win for consumers and the tech industry, but said that without knowing more about the method used by the FBI, it was hard to know what the security implications might be for iPhone users.

Cybersecurity, he said “is an arms race,’’ and the best way to keep phones secure against hackers is for users to update their devices.

The government’s silence on how it cracked the iPhone underscores a reality for the technology debate swirling around Apple and other companies: It is in law enforcement’s best interests to find and exploit vulnerabilities, but also keep that information away from the company and the public, to preserve the viability of such secret methods.

The reasons technology firms want to protect devices from infiltration are the same factors behind law enforcement’s interest in gaining access to them: smartphones are now central to how both criminals and innocent people live, a technological gateway to individual’s communications, health, financial, and other data.

Eric Berg, special counsel for Foley & Lardner and a former Justice Department attorney, said he expects technology companies to make their devices harder to crack with each iteration. In time, another case will test the issues of privacy and security again.

“It’s only a matter of time,” said Mr. Berg.

Firms increasingly have used encryption as a default setting for their products, and they have declined to help law-enforcement agencies open suspect devices in some cases.

That conflict came to a head in December, when investigators recovered the work phone of Mr. Farook after he and his wife opened fire with rifles on a holiday office party in San Bernardino, killing 14 and injuring 22. Investigators couldn’t open the iPhone because of security features that don’t allow more than 10 guesses of an iPhone’s passcode.

The Justice Department eventually got a court order compelling Apple to help them bypass the passcode security features.

The company fought the order, setting the stage for a court fight on privacy.

As the two sides geared up for that fight, FBI officials said they had exhausted all possible avenues of unlocking the phone before getting the court order against Apple.

In the public and legal debate that followed, the FBI argued the law doesn’t support a company making phones that are “warrant proof”—unable to be opened even with a signed order from a judge. Apple said it was fighting the order because to do what the FBI wanted would create a new security vulnerability for untold millions of iPhone users.

The government is still engaged in a broader fight with Apple over what role, if any, the company should play in helping investigators access data on their customers’ phones.

Previous court filings indicated prosecutors were seeking similar orders against Apple involving at least 15 phones seized as part of unrelated criminal investigations around the country.

State and local prosecutors, most notably Manhattan District Attorney Cy Vance, have also pressed technology companies to help detectives access data on suspects’ phones.