The FBI has refused to identify the make and model of the phone used by the suspected shooter, Devin Kelley, but sources familiar with the matter tell ABC News the device in question is an Apple product.
"Our team immediately reached out to the FBI after learning from their press conference on Tuesday that investigators were trying to access a mobile phone," Apple said in a statement Wednesday. "We offered assistance and said we would expedite our response to any legal process they send us.
"We work with law enforcement every day. We offer training to thousands of agents so they understand our devices and how they can quickly request information from Apple," the statement added.
On Tuesday, FBI Special Agent Christopher Combs said the agency sent a phone belonging to Kelley to its Quantico, Virginia crime lab because authorities could not unlock it. The Sunday morning attack at the First Baptist Church of Sutherland Springs left 26 people dead, according to police, who are including an unborn child in the death toll, and at least 20 others injured.
"It actually highlights an issue that you've all heard about before, the advance of the technology and the phones and the encryptions. Law enforcement, whether at the state or local or the federal level, is increasingly not able to get into these phones," Combs said. "I can assure you that we’re working very hard to get into the phone and that will continue until we find an answer.
"It could be tomorrow, could be a week, could be a month [from now]. We don't know yet, but we're going to keep working on that phone and the other digital media that we can turn it over to the rangers," he added.
Authorities have not discovered a motive for the shooting.
The FBI has criticized tech companies for how difficult it is to obtain data from locked devices even when they could contain critical evidence.
Federal authorities challenged Apple in court in the wake of the 2015 mass shooting in San Bernardino, California, in which a couple inspired by the Islamic State killed 14 people, authorities said.
ABC News' Jack Date contributed to this report.