A gunman opened fire at Oxford High School in Michigan on Tuesday afternoon, shooting 11 people. Four students were killed and several others were left critically and seriously injured.

A 15-year-old male student was later arrested in connection with the shooting and was being held at a juvenile jail, the authorities said.

The deadly gunfire in Oxford, in Oakland County — about 30 miles north of Detroit — added to a growing list of shootings on school grounds in the United States this year after a lull during the coronavirus pandemic, when many schools held classes remotely.

The shooting was the deadliest this year, according to Education Week, which tracks such shootings and reported that there had been 28 so far in 2021.

Many details, including the name of the suspect and a motive, remained unclear. Here’s what we know.

What happened at the school?

At 12:51 p.m. on Tuesday, the authorities received the first of more than a hundred 911 calls about the shooting.

The suspect emerged from a bathroom and started firing at students in the hallway, the authorities said on Wednesday after viewing security camera footage of the attack.

When students at Oxford High School first heard the gunshots, they rushed for cover and used chairs to barricade themselves behind classroom doors. Within five minutes, the authorities said, 11 people had been shot.

“I was just kind of sitting there shaking,” said Dale Schmalenberg, 16, who said he was in calculus class when his teacher heard a gunshot and locked down the classroom. “I didn’t really know how to respond.”

The gunman fired 15 to 20 shots with a semiautomatic handgun before being apprehended by deputies in the school hallway, the authorities said. Seven bullets remained in the gun, the county sheriff, Michael Bouchard, said at a news briefing on Tuesday night.

Michael McCabe, the Oakland County undersheriff, said the suspect, who had been in class earlier Tuesday, “gave up without any problems.”

Investigators were pouring through many hours of security camera footage to track the suspect’s actions, Mr. Bouchard told reporters on Wednesday, but his targets “appeared random.” Investigators had not determined a possible motive for the shooting, which Mr. Bouchard described as “absolutely brutally cold hearted.”

Sheriff Bouchard said investigators had been told that the gunman pretended to be an officer in order to access barricaded classrooms. “We know by witnesses he was tugging on doors, and we know from physical evidence he shot through doors up and down more than one hallway,” the sheriff said on Wednesday.

Had there been no intervention, Sheriff Bouchard said, “we might be, if it’s possible, in a worse situation.”

What was the aftermath?

After the shooting, students said they fled from the building. Some parents rushed to a nearby grocery store to reunite with their children.

Officials across the country expressed shock and issued statements of sadness and frustration, as residents of Oakland County announced a vigil and prepared for funerals.

Democratic leaders renewed their calls for more to be done to reduce gun violence.

“No one should be afraid to go to school,” Gov. Gretchen Whitmer of Michigan said in a statement. Later, at a news conference, she added. “I think this is every parent’s worst nightmare.”

What do we know about the victims?

The authorities on Tuesday identified those killed as Hana St. Juliana, 14; Madisyn Baldwin, 17; and Tate Myre, 16, who died in a sheriff’s squad car while on the way to a hospital. A fourth student, Justin Shilling, 17, died on Wednesday morning at McLaren Oakland Hospital in Pontiac, Mich.

“There was no time to wait,” Sheriff Bouchard said of the deputy who put Tate, who had recently won honors as a linebacker and tight end on his football team, into his car.

On Tuesday night, more than 25,000 people had signed a petition online to rename the school’s stadium after Tate, who had recently earned an all-region award from the Michigan High School Football Coaches Association.

Six other injured students ranged in age from 14 to 17, officials said, including at least two who were in critical condition and another who was in serious condition. They included a 15-year-old boy who had been shot in the head, a 17-year-old girl who had been hit in the chest and a 14-year-old girl who had been wounded in the chest and neck.

The wounded 14-year-old girl was on a ventilator Tuesday night. A hospital spokeswoman said Wednesday morning that the girl remained in critical condition, but had no further updates.

“It’s looking very tough for this young girl,” Sheriff Bouchard said.

The only adult who was injured, an unidentified 47-year-old teacher whose shoulder was grazed by a bullet, had been discharged from the hospital, he added.

What do we know about the suspect?

Few details are known about the arrested student, whom the authorities have described as a 15-year-old sophomore at Oxford High School and a resident of Oxford Village, a suburb with a population of fewer than 3,500 people.

When the boy’s parents went to a sheriff’s substation after the shooting, they declined to let investigators question their child, Undersheriff McCabe said. Officials served a search warrant on Tuesday night at his family’s house and investigators were examining the contents of his computer and phone, including social media posts. The family has hired a lawyer, officials said.

The suspect was being held at a juvenile jail Wednesday, awaiting possible charges.

“The person that’s got the most insight into the motive is not talking,” Sheriff Bouchard said at the news briefing.

He added that the 9-millimeter Sig Sauer handgun used in the shooting had been bought by the suspect’s father four days earlier. The gun had 15 round magazines, he said, two of which had so far been found at the school.

The authorities said that they did not believe that the student had planned the shooting with anyone else and that they were still investigating.

Mass shootings at schools have been a recurring tragedy in the United States. In 2018, a gunman killed 17 people and injured 17 others at Marjory Stoneman Douglas High School in Parkland, Fla. That same year, a gunman killed 10 people at Santa Fe High School in Texas.

“This is a uniquely American problem that we need to address,” Ms. Whitmer said on Tuesday.