Police in Arkansas wish to unlock an iPhone and iPod belonging to two teenagers accused of killing a couple, according to the Associated Press (AP).
Less than a day after its San Bernardino announcement, the FBI agreed to help in the homicide case.
Syed Farook and his wife were shot dead after killing 14 people in December.
A judge agreed to postpone the Arkansas case on 28 March to allow prosecutors to ask the FBI for help.
Hunter Drexler, 18, and Justin Staton, 15, are accused of killing Robert and Patricia Cogdell at their home in Conway, Arkansas last July.
Staton had been raised as a grandson by the Cogdells.
"We always appreciate [the FBI's] cooperation and willingness to help their local law enforcement partners," Faulkner County prosecuting attorney Cody Hilland told the AP.
An attorney for Mr Drexler told the AP he was "not concerned" about any data on the phone.
Previously, the FBI had asked Apple to write new software that would allow it to gain access to Farook's iPhone.
However, the company refused, arguing it would be wrong to compromise the security of one of its devices since that might lead to vulnerabilities in many other iPhones.
The FBI has offered to unlock another iPhone for police after revealing it could access the handset used by San Bernardino killer Syed Farook.