The gunman accused by US officials of murdering five US service personnel in Tennessee texted his friend a link to an Islamic verse the night before the attack, according to a report.
The text from Muhammad Youssuf Abdulazeez, 24, contained a link to Hadith 38 that included the line: "Whosoever shows enmity to a friend of mine, then I have declared war against him," according to the Reuters news agency.
The friend, who requested anonymity, received the text just after 10pm on the night before the shooting. He said he thought nothing of it at the time, but now wonders if "it may have been his way of telling me something".
Four US marines were killed on 16 July after the suspect, a Kuwait-born naturalised US citizen, sprayed gunfire at a recruitment centre in Chattanooga before driving 10km (six miles) to a Navy and Marine reserve centre where he shot and killed the victims.
Three other people were injured, including a sailor who died of his wounds in hospital on 18 July.
The suspect Abdulazeez, who was later killed in a shootout with police, also spoke out against the Israeli bombing campaign in Gaza and the civil war in Syria to his friends after a trip to Jordan in 2014.
"That trip was eye-opening for him. He learned a lot about the traditions and culture of the Middle East," one friend told Reuters.
"He felt Jordan, Qatar, Saudi Arabia were not doing enough to help, and that they were heavily influenced by the United States."
Another friend said: "He had always talked about it, but I'd say his level of understanding and awareness really rose after he came back."
After returning from his trip, Abdulazeez reportedly purchased three assault rifles on the internet and used them for target practice.
Earlier, the US Navy released a statement confirming the death of Navy Petty Officer 2nd Class Randall Smith, who succumbed to his injuries in hospital.
"Logistics Specialist 2nd Class Randall Smith succumbed to wounds received in the July 16 shooting at the Navy Operational Support Center (NOSC) in Chattanooga, Tennessee July 18 at 2:17am," it said.
US authorities are investigating the attack as an act of domestic terrorism.