Apple had been resisting a court order issued last month requiring the firm to write new software to allow officials to access Rizwan Farook's phone.
But officials on Monday said that it had been accessed independently and asked for the order to be withdrawn.
Rizwan Farook and his wife killed 14 people in San Bernardino in December.
They were later shot dead by police. US officials said Mr Farook's wide, Tashfeen Malik, pledged allegiance to the so-called Islamic State on social media on the day of the shooting.
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.
(BBC News)
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