The suspect, a student at the school in Perry, died of what investigators believe is a self-inflicted gunshot wound, an Iowa Division of Criminal Investigation official said. Authorities said one of the five people wounded was an administrator, and later identified as Perry High School Principal Dan Marburger by an eastern Iowa school district where he graduated from high school.

Authorities identified the shooter as Dylan Butler, 17, and provided no information about a possible motive. Two friends and their mother who spoke with The Associated Press said Butler was a quiet person who had been bullied for years.

Perry has about 8,000 residents and is about 40 miles (65 kilometres) northwest of Des Moines, on the edge of the state capital's metropolitan area. It is home to a large pork-processing plant, and low-slung, single-story homes spread among trees now shorn of their leaves by winter. The high school and middle school are connected, sitting on the east edge of the town.

Authorities said the shooter had a pump-action shotgun and a small-calibre handgun. Mitch Mortvedt, the state investigation division's assistant director, said during a news conference that authorities also found a "pretty rudimentary" improvised explosive device and rendered it safe.

The suspect's motive is being investigated and authorities are looking into "a number of social media posts" he made around the time of the shooting, Mortvedt added.

Sisters Yesenia Roeder and Khamya Hall, both 17, said alongside their mother, Alita, that Butler was bullied relentlessly since elementary school, but it escalated recently when his younger sister started getting picked on, too. Officials at the school did not intervene, they said, and that was "the last straw" for Butler.

"He was hurting. He got tired. He got tired of the bullying. He got tired of the harassment," Yesenia Roeder Hall, 17, said. "Was it a smart idea to shoot up the school? No. God, no."

Perry High School senior Ava Augustus said she was awaiting a counsellor in a school office when she heard three shots. Unable to flee through a small window, she and others barricaded the door and were ready to throw things if necessary.

"And then we hear 'He is down. You can go out,'" Augustus said through tears. "And I run and you can just see glass everywhere, blood on the floor. I get to my car and they are taking a girl out of the auditorium who had been shot in her leg."

Three gunshot victims were being treated at Iowa Methodist Medical Centre in Des Moines, a spokesperson said. Others were taken to a second Des Moines hospital, a spokesperson for MercyOne Des Moines Medical Centre confirmed.

Mortvedt said one person was in critical condition but the injuries did not appear to be life-threatening. The other victims were stable, he said.

Community members gathered for a vigil on Thursday evening at a park. A post on the high school's Facebook page said it would be closed on Friday and counselling services would be available for students, faculty and members of the community.

Governor Kim Reynolds ordered all flags in Iowa lowered to half-staff.

"This senseless tragedy has shaken our entire state to its core," she said.

In Washington, President Joe Biden and US Attorney General Merrick Garland were briefed on the shooting. FBI agents from the Omaha-Des Moines office are assisting with the investigation led by the Iowa Division of Criminal Investigation.

The shooting occurred in the backdrop of Iowa's looming first-in-the-nation presidential caucuses. GOP candidate Vivek Ramaswamy had a 9 am campaign event scheduled in Perry about one-and-a-half miles (2.4 kilometres) from the school but cancelled it to hold a prayer and intimate discussion with area residents.

Mass shootings across the US have long brought calls for stricter gun laws from gun safety advocates, and Thursday's did within hours. But that idea has been a non-starter for many Republicans, particularly in rural, GOP-leaning states like Iowa.

As of July 2021, Iowa does not require a permit to purchase a handgun or carry a firearm in public, though it mandates a background check for anyone buying a handgun without a permit.

Ramaswamy said the shooting is a sign of a "psychological sickness" in the country. In Des Moines, GOP rival and Florida Governor Ron DeSantis said gun violence "is more of a local and state issue" in an interview with the Des Moines Register and NBC News.

The high school in Perry is part of the 1,785-student Perry Community School District. The town is more diverse than Iowa as a whole. Census figures show 31 per cent of the residents are Hispanic, compared to less than 7 per cent statewide. Those figures also show nearly 19 per cent of the town's residents were born outside the US.

Authorities said an active shooter was reported at 7:37 am on Thursday and officers arrived within minutes. Emergency vehicles surrounded the complex.

"Officers immediately attempted to locate the source of the threat and quickly found what appeared to be the shooter with a self-inflicted gunshot wound," Mortvedt said.