An Elmwood Park elementary school reopened Monday after closing twice last week when students and staff fell ill.

Air quality experts deemed it safe for students and staff to return to Gantner Avenue Elementary School, but it remains unclear what sickened at least 11 people.

At least two students fainted, and several others fell ill last week at the school due to an unknown gas, according to school officials.

Administrators initially said they suspected a glycol leak was to blame.

Air quality tests conducted on Friday did not detect carbon monoxide, hydrogen sulfide or other combustible gases, officials said.

However, some parents claimed their children tested positive for carbon monoxide exposure. Carbon monoxide is an odorless and colorless gas.

A school board meeting will be held Tuesday, and parents told News 12 New Jersey they will be demanding answers.