Originally Published: 20 OCT 22 18:01 ET

Updated: 21 OCT 22 13:55 ET

By Steve Almasy, Jason Hanna, John Murgatroyd and Devon M. Sayers, CNN

(CNN) -- Investigators believe a 15-year-old boy killed his older brother before continuing a shooting rampage that killed four others and injured two last week in North Carolina's capital, according to a preliminary report from police released Thursday.

Authorities have not yet learned the suspect's motive in the October 13 killings of five people in the area of a Raleigh neighborhood. Police found him with a gunshot wound and he is hospitalized in critical condition, according to the city manager's preliminary report to the police chief.

The report names neither the suspect nor a 16-year-old that police believe was the first victim. But their parents have identified the suspect as Austin Thompson, and the teenage victim as his brother, James Thompson.

The suspect has not been charged. However, Wake County District Attorney Lorrin Freeman has said her office intends on charging the 15-year-old as an adult.

Based on investigators' beliefs about the suspect's direction of travel -- from a home in the Hedingham subdivision, through the neighborhood and then onto a greenway -- they believe the 16-year-old was attacked first, the preliminary report reads.

The 16-year-old, found dead in a home in the subdivision, was shot once and stabbed several times, according to the report.

Investigators believe the suspect then wounded Marcille Gardner, 59, who was found in her driveway in the subdivision. The suspect then fatally wounded Nicole Connors, 52, and killed her dog on their porch, and shot and killed an off-duty Raleigh police officer, 29-year-old Gabriel Torres, in his personal car as he was leaving for work, according to the report.

The suspect ran to the greenway where he killed Mary Marshall, 35, who was walking her dog, and Susan Karnatz, 49, who was out for a jog, according to the report.

"Based on information currently available, there does not appear to be any connection between the victims that were shot by the suspect prior to his encounter with the police other than they lived in the same neighborhood," the report adds.

About 90 minutes after the first 911 calls, police found the suspect in an area with two outbuildings on the property, the report says. An officer who was shot when the suspect opened fire from one of the outbuildings was the last gunshot victim.

Police returned fire and rescued the officer, the report says, which added that Raleigh officers fired 23 shots.

Officers then established a perimeter "due to concerns about the nature of the weapons he possessed to include multiple firearms, as well as possible hand grenades as reported in a 911 call."

When the police moved in about three hours later, they found the suspect with a shotgun and shells nearby, according to the report. He had a handgun in his waistband and an empty knife sheath clipped to his belt. A knife was recovered at the front of the building, the report adds.

The suspect was shot once, the report says.

The two Raleigh police officers who fired their guns are on administrative duty, according to the report.

The report, which city officials typically file five days after a shooting in which law enforcement is involved, also says police will petition to have the "agency recordings" of the two officers who used the weapons and the officer who was wounded during the search. In North Carolina, a judge must rule on the release of body camera videos.

The suspect's parents released a statement Wednesday through their attorneys saying they're devastated by the loss of innocent lives, including the death of their son James.

"Our son Austin inflicted immeasurable pain on the Raleigh community, and we are overcome with grief for the innocent lives lost," Alan and Elise Thompson said in the statement, which was obtained by CNN affiliate WTVD. "Words cannot begin to describe our anguish and sorrow."

The suspect's parents say they don't understand why the mass tragedy happened.

"We have so many unanswered questions," the Thompsons' statement reads. "There were never any indications or warning signs that Austin was capable of doing anything like this."

The parents said they will continue cooperating with law enforcement "and do whatever we can to help them understand why and how this happened."

According to the non-profit Gun Violence Archive, there have been at least 546 mass shootings in the United States this year as of Tuesday. That's an average of more than 1.8 mass shootings every day.

Both the Gun Violence Archive and CNN define a mass shooting as one in which at least four people are shot, excluding the shooter.