The Saudi gunman who killed three sailors at a Florida naval base had lodged an official complaint against one of his instructors, reports say.
Mohammed Alshamrani is said to have been “infuriated” after the instructor referred to him using a disparaging nickname in front of 10 other students.
The FBI says it is treating Friday’s attack, in which the gunman was shot dead, as a presumed terrorist attack.
Florida’s governor, Ron DeSantis called for more stringent security measures.
Mr DeSantis said the gunman had “a major social media trail”.
“This guy was somebody who just had a deep-seated hatred for the United States and that was pretty clear from that,” he said.
“My view is that… for us to be bringing in these foreign nationals, you have to take precautions to protect the country.”
The FBI is looking into whether Alshamrani acted alone or acted as part of a group.
Special agent Rachel Rojas said other Saudi students at the Pensacola base had been questioned but not arrested. They were reportedly confined to the base and are co-operating with investigators.
The instructor had referred to Alshamrani as “Porn Stash”, the New York Times reports, quoting communication between people involved in flight training.
“I was infuriated as to why he would say that in front of the class,” he is said to have written in his complaint.
The jibe is understood to have suggested his moustache resembled those worn by some porn actors.
Ms Rojas said Alshamrani, 21, had bought his weapon, a 9mm handgun, legally in the US.
It has been reported in US media that Alshamrani played mass-shooting videos to others at a dinner earlier in the week, according to an anonymous official briefed on the investigation.
A Twitter user appearing to match Alshamrani’s identity also made a series of anti-US posts before the shooting, an online monitoring group says.
Who were the victims?
The victims of the attack have been named as Joshua Kaleb Watson, 23, Mohammed Sameh Haitham, 19, and Cameron Scott Walters, 21.
The US Navy said the sailors “showed exceptional heroism and bravery in the face of evil”.
Why were Saudis at the US base?
The Pensacola base has long offered aviation training to foreign military forces.
Saudi pilots started training there in 1995, alongside other personnel from Italy, Singapore and Germany.
Some 200 international students are enrolled in programmes there. According to its website, the base employs more than 16,000 military and 7,400 civilian personnel.
Alshamrani was a second lieutenant in the Saudi Air Force.
Earlier, US Defence Secretary Mark Esper ordered a review of the screening process for foreign military personnel in the US.
Mr Esper told Fox News Sunday he had instructed top defence officials to look into security measures at bases.
President Donald Trump has also pledged to review foreign military programmes.
What happened on Friday?
Authorities were alerted to the shooting at the Pensacola base at 06:51 (11:51 GMT).
It took place across two floors of a classroom building and ended when a sheriff’s deputy killed Alshamrani.
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Eight people were also injured in the shooting, including two officers, who are expected to recover.
Family members of Joshua Kaleb Watson said he was shot several times but made it out of the building to alert first responders.
On Facebook, his brother Adam Watson wrote: “He died a hero and we are beyond proud but there is a hole in our hearts that can never be filled.”
Capt Tim Kinsella, the base’s commanding officer, said of the three sailors killed: “When confronted, they didn’t run from danger; they ran towards it and saved lives.”
Saudi Arabia is a key US ally in the Middle East and President Trump said the Saudi king called him after the attack to “express his sincere condolences and give his sympathies to the families and friends of the warriors who were killed”.
Questions over the US-Saudi relationship
Just as he did in the wake of the murder of the dissident Saudi journalist Jamal Khashoggi in his country’s consulate in Istanbul, President Trump is seeking to play down the significance of the Pensacola shooting.
For a president who largely sees foreign policy in transactional terms, Saudi Arabia is a great customer for the US. Billions of dollars in weapons sales translates into more than 5,500 temporary visas being issued to the Saudi military to come to the US.
Saudi personnel make up about 16% of the foreign personnel being trained at US military schools and other facilities. The Pensacola shooting is already raising questions about the vetting of these individuals.
But more broadly it once again highlights the long-standing relationship between Washington and Riyadh – a relationship that many critics in Congress see as increasingly dubious in the wake of the Khashoggi murder and the Saudi military onslaught in Yemen.