Here's what you need to know about Mohammad Youssef Abdulazeez, the gunman who opened fire at two military facilities in Tenn., killing four Marines. (Alice Li/The Washington Post)

The 24-year-old gunman who killed four Marines here on Thursday struggled to find meaning in a life that included both the outward success of an assimilated, college graduate and the turmoil of a dysfunctional, sometimes violent home.

FBI investigators spent much of Friday morning combing through and carting off material from the home where Mohammad Youssef Abdulazeez spent much of his life.

Abdulazeez, who was killed during the attack on two military sites, described his existence in an online blog as a “prison” of monotony and routine. He praised early followers of Islam’s prophet Muhammad, who “fought Jihad for the sake of Allah,” hailing them as among “the best human beings that ever lived.”

His neighbors and friends described a different Abdulazeez. He was a friendly kid, who raced go-carts down his subdivision’s winding streets and attended the neighborhood’s Fourth of July picnics with his mother and sisters. He wrestled at nearby Red Bank High School and went off to a nearby college, but still came back to the school to work and train with younger wrestlers.

Family court documents, filed by Abdulazeez’s mother in 2009 as part of preliminary divorce proceedings, describe yet another existence. Rasmia Ibrahim Abdulazeez accused the shooter’s father, Youssuf Saed Abdulazeez, “of repeatedly” beating her in the presence of their five children.

The elder Abdulazeez “struck his children without provocation” and sexually abused his wife on at least two occasions, the filing states. The elder Abdulazeez’s brothers rushed to the house from Kuwait and the District in an effort to save the marriage of 28 years, but the father’s abuse only grew worse, according to the court documents.

Still, Abdulazeez’s mother decided to drop the divorce proceedings.

As investigators combed through Abdulazeez’s life, one of their goals was to piece together the path that seemingly led Abdulazeez from the life of a normal college student to radical jihad and to a shooting rampage. “The FBI made clear yesterday that they are looking at a variety of possible motives, including the possibility of domestic terrorism,” said White House press secretary Josh Earnest. “So that is a part of their ongoing investigation.”

Neighbors, friends and co-workers were asking similar questions. Some wondered about his frequent trips to Jordan in recent years, where he said he was visiting his grandmother and an uncle. He made such four trips, including one between April and November 2014, according to U.S. law enforcement sources. An official said there was no information that the trips were connected to attempts to enter Syria or establish contact with a terrorist group.

Others questioned the influence of his father, who was frequently gone from the home and seemed distant and severe.

Here are the stories of those who died in Chattanooga

The FBI investigated the elder Abdulazeez twice, before and then just after the 9/11 attacks, for making financial contributions to a charity allegedly associated with a radical Palestinian terror group, U.S. officials said. He was put on the watch list after the second investigation, and later was removed. Numerous other Palestinian Americans were investigated and cleared for making similar contributions.

In the divorce documents, the father was quoted as saying that he intended to take a second wife, as permitted “under Islamic law” and “in the parties’ native State of Palestine.”

“Mohammad’s mother is so sweet. My mom is good friends with his mom,” said Mekenzie Abed, 21, who grew up across the street from Abdulazeez. “His dad, when you would see him, wouldn’t even make eye contact with women.”

The area where Abdulazeez grew up was the kind of place where neighbors often asked after each other’s children, and most assumed that there were few, if any, real secrets. When Abdulazeez as arrested for driving under the influence in April, Abed recalled that her mother showed him Abdulazeez’s smirking mug shot online.

“I didn’t think much of it,” Abed recalled. “I figured he’d just done a little too much partying. It happens.”

According to the SITE Intelligence Group, which monitors online postings by radical Islamist organizations, Abdul­azeez was the author of “MYABDULAZEEZ,” which was first posted on July 13 and included two entries.

In the first, “A Prison Called Dunya,” Abdul­azeez described everyday life as a prison and the Koran as a means of transcending it. In Arabic, “Dunya” refers to earthly concerns as opposed to spiritual ones.

“This life we are living is nothing more than a test of our faith and patience,” he wrote. “It was designed to separate the inhabitants of Paradise from the inhabitants of Hellfire ... Don’t let the society we live in deviate you from the task at hand.”

He added, “Brothers and sisters don’t be fooled by your desires, this life is short and bitter and the opportunity to submit to allah may pass you by.”

In his second posting, Abdul­azeez discussed the Sahaba — companions of the prophet Muhammad — and how they served their faith by bringing it to the world, sometimes through warfare.

“Every one of them fought Jihad for the sake of Allah,” he wrote. “Every one of them had to make sacrifices in their lives. . . . After the prophets, they were the best human beings that ever lived.”

Neither posting included any specific call for violence or any signal of the deadly shooting spree that would follow.

Abdulazeez, a Kuwait-born U.S. citizen, and his conservative Muslim family stood out in Chattanooga. Their subdivision consists of well-kept homes, with neatly manicured lawns shaded by towering oak trees. Abdulazeez’s family home, with its peeling paint, warped clapboards and knee-high weeds, is unmistakably different.

Some neighbors assumed the rundown home and overgrown lawn was the product of cultural differences. “They don’t have lawns in the Middle East,” said a next-door neighbor. Others wondered if it stemmed from the father’s money problems. A financial interest in a failing restaurant outside of Atlanta forced Abdulazeez’s father to declare bankruptcy in 2002. At the time, he was making about $23,000 a year as a state pesticide inspector and had debts in excess of $215,000, according to the documents.

For all the parents’ problems, the children seemed to be doing well. A sister, Dalia, taught fourth grade at Woodmore Elementary School. In 2009, the staff and student body surprised her with a schoolwide assembly to celebrate her naturalization as a U.S. citizen, singing, “My Country, ’Tis of Thee.”

She later quit her job, telling colleagues she was leaving the country to marry a man, over her conservative parents’ objections.

Another, Yasmeen, proudly wore a traditional Muslim head-scarf in class and on the volleyball court at Red Bank High School. She worked part time at a sporting goods store near her home.

Some students harassed her, but she didn’t let them intimidate her, she said in a 2010 interview with the Chattanooga Times Free Press. “I’m not afraid to go straight toward them and ask them, ‘Do you really know what Islam is?’ ” she said. “There’s this misconception that Islam is a violent religion. Muslims are actually peaceful.”

Abdulazeez, who also played soccer, was sometimes called “Abdoozy” by his jock friends. His senior yearbook entry featured two photographs of him — clean-shaven with close-cropped hair — alongside a prophetic quotation: “My name causes national security alerts. What does yours do?”

After high school, he briefly fought on the mixed-martial-arts circuit, training at the Chattanooga Fight Factory, a local gym. A video of one cage fight from 2009 shows him pummeling a middleweight from Shelbyville, Tenn., winning in the second round in a technical knockout.

Two martial arts trainers who helped prepare Abdulazeez for his fight said his father was furious afterward, apparently believing that the sport was un-Islamic. “Mohammad said he got in a lot of trouble,” said one trainer, Scott Schrader, a co-owner of the Fight Factory. “After that, I maybe saw him one or two times, and he was gone.”

To his high school friends, Abdulazeez seemed like a success. He earned an engineering degree from the University of Tennessee at Chattanooga in 2012. He traveled overseas, spending as long as seven months in Jordan during one stretch.

Sometimes, he grew out his beard, in the style of an observant Muslim. At other times, friends would run into him around town when he was clean-shaven. After bouncing from job to job, he had recently taken a position with a technology-manufacturing company in Franklin, Tenn., about two hours from his home.

He’d only been there for about three months as a shift supervisor, and appears to have skipped work there earlier this week. Several of his neighbors on his street noticed him around the house in the days before the shooting.

He was spotted stomping around in the thick, overgrown woods behind his home with an unfamiliar friend. Others saw him playing soccer on Sunday. Earlier this week, one of his neighbors noticed he was in a new rental car, instead of the beat-up vehicle he normally drove.

Justin Wm. Moyer, William Branigin, Carol D. Leonnig, Alice Crites, Julie Tate, Jennifer Jenkins, Craig Whitlock and Antonio Olivo contributed to this report.