FORT LAUDERDALE, FLA. -- Investigators say that when they captured Marine veteran Bryan Riley outside the Lakeland, Florida, home where he allegedly killed a couple, their 3-month-old son and the boy's grandmother, he told them, "You know why I did this."

But they say they don't and, in fact, may never know why Riley invaded the home of a family he had no known connection with, except that he may have been mentally ill. Riley's girlfriend told investigators that he had been saying he could communicate directly with God.

"The big question that all of us has is, 'Why?"' local prosecutor Brian Haas said. "We will not know today or maybe ever."

Riley, 33, is being held without bond on four counts of first-degree murder. During his first court appearance Monday, he said he intended to hire a lawyer; a public defender was appointed to represent him until he does.

Riley, who served in Iraq and Afghanistan, surrendered Sunday morning after a furious gun battle with authorities. A deputy then rushed into the home and rescued an 11-year-old girl who had been shot repeatedly. She was in stable condition on Tuesday morning, the sheriff's office said. A family statement said she has already undergone four surgeries to repair 10 wounds, but is alert, reading, writing and generally in good spirits.

"She was very scared when this happened, but she just prayed during the event and knew it would be OK," reads their statement to WTVT-TV. "We appreciate all the well wishes, kind words and prayers."

No one answered Tuesday at Riley's home in the Tampa suburb of Brandon. The house was already decorated for Halloween, with a scarecrow and jack-o'-lanterns. Neighbours said Riley was quiet and didn't socialize, but last week he stopped John Morris' wife, who is battling cancer, and said, "I want to pray for you."

"It was odd. He didn't make it loud or anything. He didn't talk to us much and then all of a sudden he comes up with this," said Morris, 77.

According to Polk County Sheriff Grady Judd, Riley told interrogators that the victims "begged for their lives, and I killed them anyway."

"He is evil in the flesh," Judd said. "Just because you have mental health issues, does not mean you are not criminally liable."

Judd identified Justice Gleason, 40, as one of the victims. Citing a state privacy law, Judd only identified the other victims as a 33-year-old woman, her infant son and the boy's 62-year-old grandmother. Facebook posts and public records show Gleason was in a relationship with Theresa Lanham and they had a baby boy, Jody, in May. Lanham's mother, Catherine Delgado, owned the property and lived there. Gleason also had an 11-year-old daughter from a previous relationship.

According to Judd and court records, on Saturday evening, about nine hours before the attack, Riley stopped his truck and confronted Gleason as he mowed his lawn. Riley, who lives 30 miles away, told Gleason that God had sent him to prevent a suicide by someone named Amber.

Gleason and one of the other victims told Riley no one by that name lived there and asked him to leave. They called 911, but when authorities arrived, Riley was gone. A deputy searched the area, but didn't find Riley.

Riley returned around 4:30 a.m. Sunday, arranging glowsticks to create a path leading to the house in what Judd said may have been an attempt to draw officers "into an ambush."

Shooting soon began -- a neighbour's security camera recorded at least four short volleys of about six to 10 shots each. A deputy in the area heard the gunfire and sounded the alarm, bringing state and local law enforcement officers to the scene. When they arrived, they found an apparently unarmed Riley outside, dressed in camouflage, and his truck ablaze.

But Riley ran back into the house, where authorities heard more gunfire, "a woman scream and a baby whimper," Judd said.

Officers tried to enter, but the front door was barricaded. Judd said when they went to the back, they saw Riley, who appeared to have donned full body armour.

Riley and the officers exchanged heavy gunfire, with dozens "if not hundreds of rounds" fired, before Riley retreated back into the home, Judd saidand .

Everything fell silent, until a helicopter unit noticed that Riley was coming out, the sheriff said. He had been shot once and was ready to surrender.

Officers heard cries for help inside but were unsure whether there were additional shooters and feared the home was booby-trapped. Still, one officer rushed in and grabbed the wounded girl, who told authorities there were three dead people inside.

The sheriff's office said they all had been huddling in fear, with the boy dying in his mother's arms. Even the family dog was shot dead.

"I will never be able to unsee that mother with that deceased infant in her arms," Judd said. "It is a horror of the utmost magnitude."

Authorities said Riley's girlfriend told investigators he was never violent but had become increasingly erratic. She said he claimed to be on mission from God, stockpiling supplies for Hurricane Ida victims including US$1,000 worth of cigars.

Riley's vehicle had also been stocked with bleeding control kits and other supplies for a gunfight, authorities said.

He worked as a private security guard and had no criminal history, the sheriff said.

Officers took Riley to jail in a white jumpsuit later Sunday. He appeared downcast, hanging his head and hardly opening his eyes, and didn't respond when reporters asked why he killed the family.

"This guy was a war hero. He fought for his country," Judd said. Now, "he's a cold-blooded killer."

------

Spencer, Frisaro  and Associated Press reporter Kelli Kennedy reported from Fort Lauderdale, Florida. Anderson reported from Brandon, Florida