One adult and 26 students fell ill "almost simultaneously" at Cardinal Gibbons High School in Fort Lauderdale, Florida, on Monday, authorities said. Officials said the incident took place after the students were allegedly exposed to an unknown substance in the gymnasium.

More than 100 students and faculty were in the gym for a Thanksgiving prayer before the incident occurred. Officials said the area of concern was isolated and the school was placed on lockdown. All patients were medically cleared and released from the hospital, fire officials said Monday night.

"These students were having various states. Some were having syncopal episodes, were passing out. Some had nosebleeds, some were having shortness of breath," Fort Lauderdale Battalion Chief Stephen Collan said. 

Stephen Golan, a spokesman for the Lauderdale Fire Department, also said: “At this time we have no real idea on what it was. We have searched back and forth and done several samplings and used monitors and have come up with nothing."

Principal Paul Ott said the gym was evacuated and a Hazmat team inspected the building several times, but found no contaminants. According to reports, the students began getting sick in classrooms and other areas in the school.

"They were like passing out and stuff like that," student Gustavo Hernandez told CBS News. "Some people started having nosebleeds. Some of the people were having ear pain."

After the parents learnt about the incident they rushed to the school.

"I texted with my children and one said she felt sick later on, so I told her to go to the paramedic," parent J.B. Perdomo reportedly said.

"We are trying to connect the dots. Anything we can do to make sure this doesn't happen again. I think it's just some sick kids," Ott said. "It's a mystery I don’t know if it's the bug or flu season. There was nothing found on the school premises."