The tech giant has been fighting a court order, requiring it to write new software to get into the encrypted handset of Rizwan Farook.
Now officials say help from another source has secured access to data on the phone.
Farook and his wife killed 14 people in California last December: They later died in a shootout with police.
The scrapping of the legal case is a victory for Apple. It did not make any immediate comment on Monday.
Big questions remain though over future access to phone data.
The tech industry says anything that helps authorities by-pass features will undermine security for users.
But the US government argues that blocking access will cripple all sorts of criminal investigations.
Tech industry leaders including Google, Facebook and Microsoft and more than two dozen other companies filed legal briefs supporting Apple.
The Justice Department received support from law enforcement groups and six relatives of San Bernardino victims.
