CHATTANOOGA, Tenn. — Muhammad Youssef Abdulazeez was supposed to appear in court next week for driving under the influence of alcohol. Instead authorities are struggling to understand what led him to a far more heinous act: the cold-blooded assault that left four marines and a sailor dead and a nation on edge about terrorism.

They still aren’t sure why he did it, or whether anyone else was involved. They have described their search through the remnants of his life as a domestic terrorism investigation, but nothing about his comings and goings had caught their attention before the rampage Thursday morning.

Adding to the muddled picture, many who knew him have described a clean-cut high school wrestler who graduated from college with an engineering degree and attended a local mosque. His friends and family swear they did not see it coming.

Abdulazeez rented a silver Mustang convertible, armed himself with an arsenal of weapons and ammunition, and made a one-man assault on two U.S. military facilities Thursday.

Four days later, disbelief had given way to commemoration Monday at the scene of the first shooting, a military recruiting center located in a strip mall. Hundreds of people — many carrying American flags and some with Confederate battle flags — gathered outside the recruiting office where the rampage began.

The windows, several of which were pocked with bullet holes after the shooting, have since been covered with plywood.

Several miles away, where five servicemen were fatally shot, yellow police tape still blocked access and law enforcement vehicles were parked nearby with lights flashing.

The shooting prompted governors in at least a half-dozen states to authorize National Guardsmen to take up arms to protect recruiting offices and installations.

The U.S. military also has outlined security upgrades for recruiting stations, reserve centers and other facilities, according to Capt. Scott Miller, a spokesman for the U.S. Northern Command, which covers military bases in North America.

Adm. William Gortney directed additional “force protection measures” in orders sent Sunday night, Miller said. He would not go into further detail in order to “protect operational security.”