CHATTANOOGA, Tenn. (AP) — 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."

Friends and family said Abdulazeez's behavior in the days and months leading up to the shooting was typical. He was seen dribbling a soccer ball in his yard. He told two longtime friends he was excited about his new job at a company that designs and makes wire and cable products.

"Everything seemed fine. He was normal. He was telling me work was going great," said one of the friends, Ahmed Saleen Islam, 26, who knew Abdulazeez through the Islamic Society of Greater Chattanooga and saw him at the mosque two or three nights before the attacks.

Neighbors recalled stories about a man who played with the neighborhood kids growing up and gave a lift to a neighbor who became stranded in a snowstorm. He owned guns and would shoot squirrels and practice on targets behind his house. He even described himself as an "Arab redneck," a person close to the family said on the condition of anonymity out of concern it would have business repercussions.

Bilal Sheikh, 25, said he saw his friend at the mosque two weekends ago, as they came to pray and as part of the services to celebrate Ramadan.

"I'm in total shock, like everyone else," Sheikh said. "He was always the most cheerful guy. If you were having a bad day, he would brighten your day."

But the person close to the family talked about a darker side of Abdulazeez. He was first treated by a child psychiatrist for depression when he was 12 or 13 years old and several years ago, relatives tried to have him admitted to an in-patient program for drug and alcohol abuse but a health insurer refused to approve the expense.

Abdulazeez had spent several months in Jordan last year under a mutual agreement with his parents to help him get away from drugs, alcohol and a group of friends who relatives considered a bad influence, the person said.

FBI spokesman Jason Pack declined to comment late Monday on the information the person provided.

A senior federal law enforcement official said Monday that though investigators are continuing to dig into Abdulazeez's background, they have not found any evidence suggesting that he was "specifically tasked" by anyone to shoot up the military site. The official spoke on condition of anonymity because the official was not authorized to comment on a pending investigation.

Meanwhile, Jordanian authorities have been questioning relatives of Abdulazeez as part of an investigation into the time he spent in the kingdom last year, a government official said Tuesday. The official, who would not elaborate on the probe, spoke on condition of anonymity because he was not authorized to discuss details of the case with the media

Also, authorities in the Gulf nation of Qatar said Abdulazeez transited through the country on his way back from his visit to Jordan but never set foot outside the airport. Media reports have suggested he also visited the small, energy-rich nation of Qatar in addition to Jordan. But Qatar's government insisted his time in the country was limited to its international airport.

Court records point to a volatile family life for Abdulazeez. His mother filed for divorce in 2009 and accused her husband of sexually assaulting her and abusing their children. She later agreed to reconcile.

A year after graduating from college with an engineering degree, Abdulazeez lost a job at a nuclear power plant in Ohio in May 2013 because of what a federal official described as a failed drug test.

Recently, Abdulazeez had begun working the night shift at a manufacturing plant and was taking medication to help with problems sleeping in the daytime, the person said, and he had a prescription for muscle relaxants because of a back problem.

Abdulazeez was arrested on a charge of driving under the influence April 20. He told a Chattanooga police officer he was with friends who had been smoking marijuana. The report said Abdulazeez, who had white powder on his nose when he was stopped, told the officer he also had sniffed powdered caffeine.

The arrest was "important" because Abdulazeez was deeply embarrassed and seemed to sink further into depression following the episode, the person said. Some close relatives learned of the charge only days before the shooting.

The family believes his personal struggles could be at the heart of last week's killings, the person close to them said.

"They do not know of anything else to explain it," said the person, who has been in contact with the family several times since the shootings.

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Associated Press writers Eric Tucker in Washington; Adam Schreck in Dubai, United Arab Emirates; and Karin Laub in Amman, Jordan; contributed to this report.