The suspect was taken into custody on Wednesday afternoon, a Broward County sheriff's spokesman said. The spokesman would not say if the suspect was a student.
"There are numerous fatalities. It's a horrific situation," Broward Country Schools Superintendent Robert Runcie told reporters.
He said the school district had no warning of a potential shooter and that there was no evidence of more than one shooter.
"We have at least 14 victims," the Broward County sheriff's office said on Twitter. It did not make clear if that number included people who were wounded, in addition to the dead.
Students hid in classrooms until they were rescued by police in tactical gear, friends and family members said.
The sound of gunfire ripped through the air shortly before students were to be dismissed for the day, school officials said on Twitter, adding that multiple people had been injured in the incident.
School officials and police asked parents who were in touch with their children to tell them to remain hidden in classrooms while police searched for the shooter.
"Police are working an active shooter scene, and ask that if you are in touch with your student you ask that they remain calm and barricaded until police come to their room, this is for everyone's safety," Coral Springs police said earlier on Twitter.
Panicked parents checked on their children.
"My daughter, as of right now, she's still trapped in a closet. She's afraid to speak," a man who identified himself as Caesar Figueroa and said his daughter was inside the school, told CBS News.
"I told her, 'Don't call me, because I don't want no one to hear your voice.' So, she's still trapped in a closet in there."
Local FOX-10 TV reported five people were seen being treated by paramedics while other students were filing out of the school in a line.
Live television showed dozens of students running and walking away from the school, weaving their way between law enforcement officers with heavy weapons and helmets, and large numbers of emergency vehicles including police cars, ambulances and fire trucks.
Florida Governor Rick Scott said on Twitter he had been in touch with local officials.
Parkland is about 24km west of Fort Lauderdale in the US state of Florida.