Apple has said that the FBI is to blame after the bureau complained it was unable to access the smartphone of Texas gunman Devin Kelley.
The California-based company alleged that the FBI did not accept its help to unlock the killer's phone during a "critical" 48-hour window after the attack.
Sunday's mass shooting at a church in Texas killed 26 worshippers – including nine members of the same family.
FBI special agent Christopher Combs, who is leading the investigation into the gunman, claimed that the agency has been unable to access an encrypted phone belonging to Kelley.
He said the inability to access the device highlighted "an issue that you have all heard about before" – alluding to an argument between the FBI and Apple following on from a mass shooting in San Bernadino.
Mr Combs said he would not mention the make of Kelley's phone specifically as he did not want to "tell every bad guy out there what phone to buy", although it has been widely reported to be an iPhone.
The FBI ultimately paid more than $1m to a non-government party to hack into the phone of the San Bernadino shooter – with Apple subsequently facing criticism over the role its technology plays in protecting subjects of investigation.
Apple disputed the FBI's account of the difficulties of accessing Kelley's phone – claiming its representatives reached out to the agency "immediately" in the aftermath of the attack but bureau operatives did not respond.
According to Reuters, which cited sources familiar with the investigation, the FBI subsequently missed the opportunity to allow law enforcement to unlock Kelley's phone using his fingerprint.
Once a 48-hour window passes, the fingerprint-unlocking mechanism is deactivated as part of an iPhone's security measures – meaning the user has to enter their passcode to unlock the device.
If Kelley used iCloud storage, investigators may be able to request data by delivering a court order or a warrant to Apple. The tech giant has complied with such warrants in the past.
It is not known whether the US Department of Justice wishes to repeat its protracted legal battle with Apple over the encryption of its devices.