A shooting outside a high school in Des Moines on Monday afternoon left a 15-year-old boy dead and two other teenagers in critical condition in what appeared to have been a drive-by attack, the authorities said.

Des Moines police officers responded at 2:48 p.m. to the shooting on the grounds of East High School, which serves about 2,000 students in Iowa’s capital city.

The teenagers appeared to have been shot with a gun fired from a passing vehicle, Sgt. Paul Parizek, a spokesman for the Des Moines Police Department, said in a news release.

Potential suspects have been detained, Sergeant Parizek said, though no charges have yet been filed. Multiple search warrants are being executed.

It was not immediately clear what had motivated the shooting at the school, one of several public high schools in Des Moines. The 15-year-old boy who was killed did not attend the high school. The two teenagers who were critically injured, 16-year-old and 18-year-old females, are East High School students, the police said.

“We know that the kids in that school are our community’s most precious cargo,” Sergeant Parizek said during the news conference. “It’s sad, but we do train for this. We have to be prepared for it, and this is why.”

Students and teachers sheltered in place while the school was placed under a lockdown that was lifted later in the afternoon. The police said that the building was secure and that there was no longer a threat.

Kaylie Shannon, a student at the school, told the television station KCCI that she was outside the school when she heard gunfire.

“I was just sitting in my friend’s car and then all of the sudden I heard 11 gunshots and some boys screaming,” she said.

The Des Moines Public Schools superintendent, Thomas Ahart, said in a statement that firearms were “far too easily accessible” in Iowa and throughout the United States, and called for “real change to gun laws and access.”

“We live in an era when shootings in and near schools have become too common,” Mr. Ahart said. “Our staff and students are forced to train for these incidents and the trauma associated with the repeated drills and incidents will remain with them for years to come.”

The Bureau of Alcohol, Tobacco, Firearms and Explosives said on Twitter on Monday that special agents from its office were on the scene and helping with the investigation.