Last week, prosecutors said “an outside party” had demonstrated a possible way of unlocking the iPhone without the need to seek Apple’s help.
A court hearing with Apple was postponed at the request of the justice department, while it investigated new ways of accessing the phone.
At the time, Apple said it did not know how to gain access, and said it hoped that the government would share with them any vulnerabilities of the iPhone that might come to light.
On Monday a statement by Eileen Decker, the top federal prosecutor in California, said investigators had received the help of “a third party”, but did not specify who that was.
Investigators had “a solemn commitment to the victims of the San Bernardino shooting”, she said.
“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 co-operation from relevant parties, or through the court system when cooperation fails,” the statement added.
Analysis: Dave Lee, BBC North America technology reporter
A court case that had the US technology industry united against the FBI has, for the time being, gone away.
Now this debate moves into more uncertain times. The US government has knowledge of a security vulnerability that in theory weakens Apple devices around the world.
To protect its reputation, Apple will rush to find and fix that flaw. Assuming it can do that, this row is back to square one.