The night before a gunman killed five US service personnel in Tennessee, he sent a text message linking to a religious verse about "declaring war", reports say.
Mohammad Youssuf Abdulazeez also spoke out against conflicts in the Middle East, according to the Reuters agency.
Police have said his motives are still being investigated.
His family said Abdulazeez, who was killed in a shootout with police, suffered from depression.
In a statement expressing "horror" at the attack, his family said: "The person who committed this horrible crime was not the son we knew and loved.
"For many years, our son suffered from depression. It grieves us beyond belief to know that his pain found its expression in this heinous act of violence."
Meanwhile, Reuters and the New York Times say they have seen a text sent the night before to a friend linking to a passage of Islamic text - Hadith 38 - containing the verse: "Whosoever shows enmity to a friend of Mine, I will indeed declare war against him."
Full text of Hadith 38
Allah the Almighty has said: "Whosoever acts with enmity towards a friend [wali] of Mine, I will indeed declare war against him. Nothing endears My servant to Me than doing of what I have made obligatory upon him to do. And My servant continues to draw nearer to Me with supererogatory [nawafil] prayers so that I shall love him. When I love him, I shall be his hearing with which he shall hear, his sight with which he shall see, his hands with which he shall hold, and his feet with which he shall walk. And if he asks [something] of Me, I shall surely give it to him, and if he takes refuge in Me, I shall certainly grant him it."
The friend, who requested anonymity, said he thought nothing of the text at the time, but now wonders if it was a hint at the attack to come.
Other friends also told Reuters that Abdulazeez spoke of his anger about conflicts in the Middle East, including Israeli bombing campaigns in Gaza and the civil war in Syria, after returning from a trip to Jordan last year.
"He had always talked about it, but I'd say his level of understanding and awareness really rose after he came back," said one of the friends interviewed.
They said he had purchased three guns online following the visit, and used them for target practice.
"It would be premature to speculate on exactly why the shooter did what he did," FBI agent Ed Reinhold said. "However, we are conducting a thorough investigation to determine whether this person acted alone or was inspired or directed."
Abdulazeez began Thursday's attack with a shooting spree at a recruitment centre in Chattanooga.
He then drove about six miles (10km) to a Navy and Marine reserve centre, where he shot and killed four marines.
A fifth victim, named as Navy Petty Officer 2nd Class Smith, died of his injuries on Saturday.
A relative told the US military newspaper Stars and Stripes that Smith was married with three young daughters.