The gunman in a deadly rampage late last year at a military base in Florida communicated with al-Qaida operatives about the attacks in the months leading up to it, U.S. officials said Monday as they laid out new details of a shooting that killed three American sailors.

The FBI learned of the contacts between Mohammed Saeed Alshamrani and operatives of al-Qaida after breaking the encryption on cellphones that had previously been locked and that the shooter, a Saudi Air Force officer, had tried to destroy before being killed by law enforcement.

“We now have a clearer understanding of Alshamrani’s associations in the years, months and days leading up to his attack,” Attorney General William Barr said at a news conference in which he sharply chastised Apple for not providing help in unlocking the phones.

Once unlocked by the FBI, the phones revealed contact between Alshamrani and “dangerous” operatives from al-Qaida in the Arabian Pensinsula, or AQAP, and also showed how he had been radicalized overseas for at least the last five years, officials said.

Law enforcement officials left no doubt that Alshamrani was motivated by jihadist ideology, saying he visited a New York City memorial to the attacks of Sept. 11, 2001, over the Thanksgiving holiday weekend and posted anti-American and anti-Israeli messages on social media just two hours before the shooting.