US citizens have been urged by the Pentagon not to carry out armed patrols outside military recruitment centers.
Civilians acting as unofficial guards have appeared outside some centers since five service personnel were shot dead last week in Chattanooga, Tennessee.
Military personnel are generally barred from carrying firearms at recruitment centers and bases.
The Pentagon says it appreciates the support but armed civilians could pose an unintended security risk.
In a statement spokesman Peter Cook said: “While we greatly appreciate the outpouring of support for our recruiters from the American public, we ask that individuals not stand guard at recruiting offices as it could adversely impact our mission, and potentially create unintended security risks.
“We continue to partner with and rely on first responders for the safety of the communities where our service members live and work.”
According to officials, the Chattanooga gunman – 24-year-old gunman Muhammed Youssef Abdulazeez – acted alone when he attacked two military facilities, killing five service members.
Muhammed Youssef Abdulazeez was shot and killed by police during the attack. His motive was unclear.
Since then, armed civilians – some of them members of private militias – have turned up outside recruitment centers saying they are supporting those inside.
One group appeared in Cleburne, Texas, armed with assault rifles and calling themselves Operation Hero Guard.
In Lancaster, Ohio, armed civilians were ordered off the property after one accidentally discharged his rifle into the pavement.
US officials say there is no indication of further danger to recruitment centers and the government does not intend to change the way they are staffed.
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.
A fifth victim has died from wounds sustained in Chattanoog attack two days ago, the US Navy announces.
The death of logistics specialist Randall Smith brings to five the number of people killed by gunman Mohammad Youssuf Abdulazeez. The other four victims were US marines.
Navy Petty Officer 2nd Class Randall Smith, 26, succumbed to wounds received in Thursday’s gun rampage, the US Naval Institute reported.
Mohammad Youssuf Abdulazeez was himself later killed in a shootout with police.
The FBI has said his motives for the shooting were unclear.
However, it says it has found no evidence that he was carrying out an international terror plot.
Mohammad Youssuf Abdulazeez first fired shots at a recruitment centre in Chattanooga, then drove about 6 miles to a Navy and Marine reserve centre, where he shot and killed the marines, and wounded Navy Petty Officer 2nd Class Smith.
“A male Navy Petty Officer succumbed to wounds received in the July 16 shooting at the Navy Operational Support Center (NOSC) in Chattanooga, Tennessee” on Saturday at 02:17 local time, the US Navy said in a statement.
It did not name the sailor, but family members confirmed it was Randall Smith.
A relative told the military newspaper Stars and Stripes that she said Randall Smith was married with three young daughters.