CHATTANOOGA (Tennessee) • Investigators believe that a 24-year-old gunman was acting alone when he opened fire at two military offices here, killing four Marines in an attack that officials said could be an act of domestic terrorism.
The gunman, who has been identified as Mohammod Youssuf Abdulazeez, is also not known to have links to terrorist groups.
Thursday's mid-morning attack was an ugly reminder of other deadly shootings at US military installations, including a 2009 rampage at Fort Hood in Texas that left 13 dead and a 2013 attack at the Navy Yard in Washington that left 12 dead.
It also came amid fears about the threat posed by "lone wolves" in the United States.
Officials said Abdulazeez first drove to a joint military recruiting centre in an open-top car, spraying it with gunfire and riddling the glass facade with bullet holes.
"Everybody was at a standstill and as soon as he pulled away, everyone scrambled, trying to make sure everyone was okay," said Ms Erica Wright, who works two doors down from the centre.
Abdulazeez then drove off to a Naval Reserve Centre about 10km away, fatally shooting the four Marines before being shot and killed in a shoot-out with police.
Three other people were injured in the shootings, which unfolded over 30 minutes.
Ms Carolyn Taylor, who works across the street from the naval centre, heard scores of gunshots.
"At least 100, at least, because it was several at one time," said Ms Taylor, who added that police officers swarmed the area, their weapons drawn, "and then, within seconds, we heard the gunfire".
Officials said Kuwaiti-born Abdulazeez became a naturalised US citizen and went to high school and college in Chattanooga.
Although counter terrorism officials had not been investigating him before Thursday's shooting, federal officials familiar with the inquiry said his father had been investigated years ago for giving money to an organisation with possible ties to terrorists. The Federal Bureau of Investigation (FBI) raided his home and vowed to use every means possible to "determine the cause or the reason why he carried out this attack". But FBI officials told a news conference that, thus far, they did not have "anything that directly ties" the young man to global terrorist organisations.
According to the Site Intelligence Group, which tracks terrorist activities, Abdulazeez this week posted at least two Islam-focused writings on a blog, including one in which he described life as "short and bitter". He also said Muslims should not miss "the opportunity to submit to Allah".
President Barack Obama called the shootings "heartbreaking" and asked Americans to pray for the relatives of the victims.
The Department of Homeland Security said that it had ordered that security be stepped up at "certain federal facilities, out of an abundance of caution".
"It is incomprehensible to see what happened and the way that individuals who proudly serve our country were treated," Chattanooga Mayor Andy Berke said, praising the quick response by law enforcement to prevent further loss of life.
The day of gunfire unnerved Chattanooga, one of Tennessee's largest cities and a place known more for its scenery and tourism than talk of terrorism and violence.
After the shootings, local universities ordered lockdowns, and officials investigated reports of gunfire at a mall in the Chattanooga suburb of Cleveland. But much of the speculation turned out to be unfounded.
REUTERS, NEW YORK TIMES, AGENCE FRANCE-PRESSE