Three young children and a teacher were killed Thursday afternoon in a horrific shooting at an elementary school in Bend, Oregon, and a teenage suspect was fatally shot by a good Samaritan who pursued him.

The shooter forced entry into Silver Rail Elementary School by firing through the locked doors around 1 p.m. PT (4 p.m. ET), Bend police said.

Police arrived within three minutes and learned the suspect had been shot and killed.

Many questions remain about the motive and how the shooter gained access to the long gun used in the attack.

Here’s what we know.

Timeline

Police arrived within three minutes of the first call for help.

Deschutes County dispatch received a call at 12:59 p.m. PT from a person in a vehicle saying they were being “shot at multiple times by a person with a long gun,” Bend Police Chief Mike Krantz said at a news conference Thursday evening.

In the ensuing minutes, dispatch received “dozens” of 911 calls reporting that a shooter had entered the K-5 school, he said.

A tone alert was issued at 1 p.m. telling all law enforcement officers to report to a critical incident.

One minute later, the school was directed to go into lockdown.

At 1:02 p.m., the first Bend police officers arrived and moments later learned that a suspect “had been shot by a community member who had seen the suspect enter the school and pursued him into the school and engaged him,” Krantz said.

Officials did not say how far the gunman got into the school, which has more than 400 students and over 40 staff members.

Officers entered the school, confirmed the suspect was dead and began to evacuate it, busing people to a reunification center at a nearby church and providing aid to the victims.

Oregon Gov. Tina Kotek praised the courage of the school’s staff members, parents and first responders and the unidentified community member who “heroically intervened to stop further loss of life.”

The victims

Three children — ages 5, 8 and 11 — and an adult teacher were killed. Their identities were not publicly disclosed, but their families were notified, Krantz said.

A 7-year-old and a 54-year-old teacher were taken to St. Charles Medical Center — Bend with critical injuries, he said.

What we know about the suspect

Officials identified the suspect as a 16-year-old Bend resident but did not disclose his name.

Authorities declined to say how many shots were fired.

A Bend resident who was walking near the school Thursday afternoon heard at least six shots ring out.

“They were spaced out, which made us question whether it was gunshots or someone doing construction,” Leah Schwartz told NBC News.

Police said they are still investigating the suspect and did not say whether he previously attended the school or knew anyone who did.

The Bureau of Alcohol, Tobacco, Firearms and Explosives received the results of an urgent trace on the long gun used by the gunman Thursday evening, a source familiar with the investigation and the crime scene said.

Authorities have not said how the shooter got access to the weapon. In Oregon, a person must be 18 to purchase rifles and shotguns and 21 for handguns. It is illegal to sell firearms to minors.

However, people under 18 can legally possess firearms if they have parental consent, in which case they may use non-handgun firearms. Minors can also temporarily possess firearms for lawful reasons, such as hunting or target practice.

The investigation

Krantz said there will be a large law enforcement presence at the school for several days.

“This is going to be a long investigation,” he said.

Kotek, the governor, said that the Oregon Department of Emergency Management is coordinating across multiple state agencies and that “the state stands ready to support” the community. The ATF and the FBI were also assisting.

All schools in the Bend-La Pine Schools district, as well as activities and athletic events, are canceled Friday.

Bend Mayor Melanie Kebler said Thursday that it was an “awful day for our community.”

“A school should be a safe place where children can learn and feel secure,” she said.