CHATTANOOGA, Tennessee - Bits and pieces have emerged over the past few days about Muhammad Youssef Abdulazeez's troubled life. But two significant pieces of the puzzle are missing: Why did he ambush two military sites, killing four Marines and a sailor? And was he propelled to do so by his own demons or at the direction of someone else?

Until last Thursday's shooting, the Kuwait-born 24-year-old was not on the radar of terrorism investigators. However, unlike situations where a suspect is known to authorities, investigators did not have a head start. As a result, a portrait of his background, contacts, computer use and travels must be assembled from the ground up and pieced together.

Abdulazeez blended into everyday life in Chattanooga as a clean-cut high school wrestler who graduated from college with an engineering degree and regularly attended a local mosque.

But he also had a more turbulent side, as evidenced by his arrest for drunken driving after returning from a visit to Jordan. He was set to face a judge later this month.

Abdulazeez was killed in a shootout with police at a Marine-Navy facility where the slain servicemen were killed. Authorities said Abdulazeez was driving a rented silver Mustang convertible, wore a vest with extra ammunition and wielded at least two long guns _ either rifles or shotguns _ and a handgun.

About seven miles (11 kilometers) away, in a small strip shopping center, hundreds of people _ many carrying American flags and some with Confederate battle flags _ gathered outside the military recruiting office where the rampage began.

The shooting prompted governors in at least a half-dozen states to authorize National Guardsmen to carry 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 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. He owned guns and would shoot squirrels and practice on targets behind his house.

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 a person close to the family, speaking on condition of anonymity, described 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.

Court records point to a volatile family life. 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, 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.

Abdulazeez was arrested on a charge of driving under the influence April 20. He told a Chattanooga 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 Thursday.