A 13-year-old boy killed Sunday night in a two-car crash in Mississauga, Ont., is being remembered as "outgoing," "bubbly" and a well-loved member of his school's basketball team.

"This tragedy has brought great sadness to the students and staff," says a letter sent home with students from Erin Mills Middle School, where Keziah Edwards-Young was a Grade 7 student.

"Keziah was a well liked student — a valuable member of our school community. We will miss him," the letter from principal Monika Hurford says.

Carla Pereira, spokesperson for the Peel District School Board, said the mood at the school is a sombre one. "There's certainly a lot of distraught students and a lot of distraught staff members," owing to how many people at the school called Edwards-Young a friend.

Flag being lowered to half-mast

Counsellors will be available at the school for as long as they're needed, said Pereira, and the flag will be lowered to half-mast in the boy's honour until after his funeral. A table at the school's library will also function as a memorial where anyone can leave messages of remembrance and condolence, the school said.

A man driving alone in one of the cars was also killed in the accident.

A two-car crash in Mississauga, Ont., late Sunday left an adult and two children dead, and three people injured. (Tony Smyth/CBC)

Three other people, including a woman and a girl, were injured in the two-car crash, which occurred on Winston Churchill Boulevard, north of Queen Elizabeth Way, just before 11 p.m. ET.

Const. Rachel Gibbs, spokesperson for Peel Regional Police, said the man who died was driving a Mazda 6 Sedan southbound on Winston Churchill Boulevard when he lost control of the vehicle for unknown reasons. The car crossed the centre line and hit a northbound Mercedes Benz.

The man, 40, was the only occupant of his car. He was pronounced dead at the scene. The boys in the Mercedes, five and 13 years old, were rushed to hospital, where they died of their injuries. Next of kin have been notified, police said.

A passenger in the Mercedes, a woman, 25, is in critical condition in hospital. Another passenger, a girl, 7, suffered a broken leg. She was rushed to Toronto's Hospital for Sick Children, where she was listed in stable condition. The driver of the Mercedes, a man, 24, suffered minor injuries.

Gibbs said police have not yet determined whether the people in the Mercedes are related to each other. Police are talking to eyewitnesses but want to reach anyone who may have seen the crash or the vehicles just beforehand.

Speed may be a factor

She could not say whether the crash was a head-on collision but said speed may be a factor in the crash. One of the cars was turned into a twisted pile of metal.

Some of the victims were ejected from the vehicles, but Gibbs couldn't say how many or whether seatbelts were being worn at the time of the crash. Some victims were seen on the ground after the collision.

Oakville fire crews search the wreckage of a car after the fatal crash on Winston Churchill Boulevard. (Tony Smyth/CBC)

Winston Churchill Boulevard, which was closed between the QEW and North Sheridan Way on Monday morning as police investigated, was reopened later in the morning.

Halton Regional Police were the first to arrive at the scene following the crash. At least two police officers performed CPR on victims before paramedics arrived. Police vehicles also escorted ambulances to hospitals.

Const. Bancroft Wright, a media relations officer with Peel police, said responding to the scene was "traumatic" for the officers.

"Those first responders also have children," he said.

Other agencies at the scene included the Oakville and Mississauga fire departments, Halton Regional Police, and paramedic services from Halton and Peel regions.

Peel Regional Police's major collision bureau has taken over the investigation.

Police are appealing for eyewitnesses to the crash. (Tony Smyth/CBC)

Winston churchill and qew..ive never heard such heartbreaking and devastating screams before :( <a href="https://t.co/bNJnnBCVxL">pic.twitter.com/bNJnnBCVxL</a> —@IllIIIIl

Video: Peel Police provide an update on triple fatal crash on Winston Churchill Blvd north of QEW. <a href="https://t.co/Xoxvtn4Qzx">https://t.co/Xoxvtn4Qzx</a> —@LateNightCam

Three confirmed fatalities, investigators still on scene, WCB between QEW and North Sheridan Way will be closed until late morning. —@PeelPoliceMedia