At least 17 people were dead after a 19-year-old man opened fire at Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday afternoon, officials said.

Fourteen others were wounded, five of whom had life-threatening injuries Wednesday night, hospital officials said. Authorities said the suspect, identified as Nikolas Cruz, concealed himself in the fleeing crowd and was arrested in nearby Coral Springs. He was treated for "labored breathing" as a precaution but was released from the hospital, they said.

"You come to the conclusion this is just absolutely pure evil," Florida Gov. Rick Scott, his hands clutched over his chest, said Wednesday night.

Little was known about Cruz on Wednesday night. Broward County schools Superintendent Robert Runcie said he had recently been expelled from Douglas for disciplinary reasons and was currently enrolled in another school in the district. The general store chain Dollar Tree confirmed that Cruz worked at its Parkland store and said it was fully cooperating with investigators.

Broward County Sheriff Scott Israel, who told reporters Wednesday night that the son of one of his deputies was among the injured, said Cruz was believed to have been armed with a AR-15-style semiautomatic rifle and multiple magazines. Israel said it was unclear whether he had any other weapons.

The gunfire began outside the school and continued inside, where 12 of the victims were killed, Israel said. All of those victims have been identified, he said, but no identities will be made public until the families of all of those affected have been notified.

Federal and local authorities told NBC News that there was no indication that the gunman had an accomplice or accomplices.

Cruz was taken into custody off campus about an hour after he "committed this horrific, detestable act," said Israel, who said investigators were reviewing social media postings that he described as "very disturbing."

Brandon Minoff, a senior at the school who said he had two classes with Cruz two years ago, told NBC News that while it was "surreal" to hear Cruz named as the suspect, "I wasn't surprised."

"I got paired with him for a project, and he started talking to me about his life — how he was held back twice, expelled from two private schools. He likes to do reckless stuff," Minoff said.

"He had aspirations to join the military," Minoff said. "He enjoyed hunting."

Sebastian Toala, another senior, told NBC Miami: "I never really got close to him, because I always had a feeling there was something wrong."

Parkland, in north Broward County, is about 30 miles northwest of Fort Lauderdale. The shooting on the sprawling campus happened despite the presence of police officers at the school.

Runcie, the school superintendent, said at last two police cars were typically on campus "on a daily basis."

While students filed out of the school with their hands up, heavily armed SWAT team members conducted a class-by-class search to make sure there were "no other shooters" — and to retrieve any bodies, he said.

Israel, who said his triplets once attended the school, urged worried parents to head to a nearby hotel to collect their children.

"This is a terrible day for Broward County, the state of Florida, the United States," he said. "There really are no words."

The first sign that something awful was happening came around 2:30 p.m., not long before classes were supposed to have been dismissed, when authorities were called to respond to an active shooter.

For more than an hour, the school was at the mercy of a gunman on the loose.

"He was outside and inside the school," Israel said.