VP Joe Biden has called Mohammad Youssuf Abdulazeez, who killed four marines and a sailor in Chattanooga in July, a “perverted jihadist”, despite no official determination of motive.
Speaking at a memorial service in the Tennessee town, Joe Biden also called the suspect a “perverse ideologue”.
According to authorities, Mohammad Youssuf Abdulazeez acted alone when he attacked two military facilities in Chattanooga.
However, the FBI has said it has not yet been able to establish a motive.
Joe Biden was joined by Defense Secretary Ashton Carter and families of the victims for the service, a month on from the July 16 attack.
The vice president alluded to the recent loss of his own son Beau Biden, a former attorney general of Delaware who died from brain cancer in May.
“I wish I were not here,” he told the families.
“I have some sense of how hard it is for you to be here.”
“I didn’t have the privilege of knowing any one of them personally but oh, I knew them,” Joe Biden added.
“Confident, determined, trustworthy, compassionate and always, always loyal. I knew them. They were my son. And so many other sons I know.”
FBI investigators say Mohammad Youssuf Abdulazeez, 24, acted alone in the crime but say they are still unsure whether he was inspired by radical ideology or driven by mental illness as his family has suggested.
“The meaning of their killing is yet unclear, what combination of disturbed mind, violent extremism and hateful ideology was at work, we don’t know,” said Ashton Carter.
Mohammad Youssuf Abdulazeez, a former engineering student, was shot dead by police after opening fire at a military recruitment centre and later a reserve centre 7 miles away.
Ashton Carter has ordered the military to review its procedures for protecting American troops inside the US and to step up security at recruiting stations.
According to new reports, the night before Mohammad Youssuf Abdulazeez killed five US service personnel in Chattanooga Tennessee, he sent a text message linking to a religious verse about “declaring war”.
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.
Mohammad Youssuf Abdulazeez, who was killed in a shootout with police, suffered from depression, his family said.
In a statement expressing “horror” at the attack, the 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.”
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 Mohammad Youssuf 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.”
Mohammad Youssuf Abdulazeez began Thursday’s attack with a shooting spree at a recruitment centre in Chattanooga.
The gunman then drove about six miles to a Navy and Marine reserve centre, where he shot and killed four marines.
A fifth victim, named as Navy Petty Officer 2nd Class Randall Smith, died of his wounds on July 18.