Story highlights Officials expect to lift the stay-indoors order at 6 a.m. local time

Three hundred gallons of hydrochloric acid leak after a tank ruptures, an official says

Four firefighters are among the nine people hospitalized for exposure, the official says

Residents of Texas City are ordered to remain indoors, emergency officials say

A toxic cloud that formed after 300 gallons of hydrochloric acid leaked at a southeast Texas storage facility sent nine people to hospitals and forced thousands of residents indoors, an emergency management official said Thursday.

Four firefighters were among those who were hospitalized for exposure after a tank ruptured at a storage facility near the Port of Texas City, Bruce Clawson of the local emergency management office said.

More than 45,000 residents of Texas City were ordered by emergency management officials to remain indoors, turn off air conditioning units and make sure all windows and doors were closed until the vapor cloud dissipated.

The order was expected to be lifted by 6 a.m. (7 a.m. ET), Clawson said.

Officials did not immediately detail what caused the tank to rupture late Wednesday at the Dallas Group of America's facility near the port. A telephone call by CNN to the New Jersey-based company early Thursday morning was not immediately returned.

City officials were working to clean up the leak, Clawson told CNN affiliate KTRK

None of those exposed to the chemical cloud sustained life-threatening injuries, according to KTRK.

It wasn't immediately known what the chemical was being used for at the facility. Hydrochloric acid has a number of industrial uses. It also has a corrosive effect on human tissue, with the potential to damage the lungs, eyes, skin, and intestines.

It is not the first time the residents of Texas City have dealt with a threat from a chemical leak.

In March, an acid leak was reported at BP's refinery in the port city. And last year, several dozen homes were evacuated when 250,000 gallons of gasoline spilled following a pipeline rupture.