Apple, backed by a broad coalition of technology giants like Google and Facebook, was fiercely opposed to assisting the US government in unlocking the iPhone on grounds it would have wide-reaching implications on digital security and privacy.
A key court hearing scheduled earlier this month to hear arguments from both sides in the sensitive case was abruptly cancelled after the FBI said it no longer needed Apple's help and had found an outside party to unlock the phone.
"Our decision to conclude the litigation was based solely on the fact that, with the recent assistance of a third party, we are now able to unlock that iPhone without compromising any information on the phone," US attorney Eileen Decker said in a statement yesterday.
"We sought an order compelling Apple to help unlock the phone to fulfil a solemn commitment to the victims of the San Bernardino shooting -- that we will not rest until we have fully pursued every investigative lead related to the vicious attack."
It was unclear who helped the FBI access the phone and what was stored on the device. But news reports have said the FBI may have sought assistance from an Israeli forensics company.
In a statement, Apple said the FBI case should never have been brought before the courts and that the company would continue to increase the security of its products.
"Apple believes deeply that people in the United States and around the world deserve data protection, security and privacy," it said. "Sacrificing one for the other only puts people and countries at greater risk."
In a court filing asking that the case be dismissed, federal prosecutors said the US government had "successfully accessed the data stored on (Syed) Farook's iPhone and therefore no longer requires assistance from Apple Inc."
Farook and his wife Tashfeen Malik killed 14 people in San Bernardino, California on December 2 before dying in a firefight with police. Two other phones linked to the pair were found destroyed after the attack.
Tech companies, security experts and civil rights advocates had vowed to fight the government all the way to the Supreme Court, saying the case was not about a single phone and could set a precedent to compel companies to build backdoors into their products.
Evan Greer, campaign director of Fight for the Future, a non-profit that supports Apple, said Monday's announcement was clear proof the government had an alternative motive in the case.