Once the FBI announced that it had unlocked the iPhone of one of the shooters involved in the terrorist attack in San Bernardino, Calif., the bureau received other requests for assistance.
Prosecutors around the country want help in opening phones that may shed light on their cases.
The FBI agreed on Wednesday to help an Arkansas prosecutor unlock an iPhone 6 and an iPod that may contain evidence in a murder case.
The devices belong to two teenagers accused of killing Patricia and Robert Cogdell in Conway, Ark., in July.
On Monday, FBI officials confirmed that the bureau, without assistance from Apple, had gained access to the phone of Syed Rizwan Farook, one of the San Bernardino shooters.
Farook and his wife died in a gun battle with police after they killed 14 people at the Inland Regional Center in December.
The FBI hasn't said how it was able to get into the iPhone 5c.
NPR tech reporter Alina Selyukh posted earlier this week: "Apple says the FBI has not shared its technique with the company ... and its lawyers have previously said they'd push for such a disclosure."
Shortly after Monday's FBI announcement, a judge in Faulkner County, Ark., agreed to delay the trial of 18-year-old Hunter Drexler, one of the teens charged in the couple's death, so prosecutors could ask the bureau for help getting access to the phone's contents.
Faulkner County Prosecuting Attorney Cody Hiland told The Associated Press that the FBI had agreed to help less than a day after the initial request.
What isn't known is whether the FBI will try to get into the devices in Arkansas the same way it did in the San Bernardino case.
Meanwhile, a district attorney in Baton Rouge, La., is waiting for word from the FBI on whether it will help unlock a phone owned by a murder victim.