A police officer told investigators Tuesday that he shot a 14-year-old boy suspected in the overnight break-in of a Houston-area classroom after believing the teenager was reaching for a gun.

The boy was in critical but stable condition at a Houston hospital and was expected to survive, authorities said. The officer was placed on leave pending the investigation.

Pasadena Police Assistant Chief Bud Corbett said the officer, a 17-year law enforcement veteran, told investigators he did not know how old the suspected burglar was when he shot him. The names of the boy and the officer were not released.

The boy, an eighth-grader at the school, was shot in the chest by the district police officer, who was investigating a broken window in a portable classroom containing computers after an alarm went off, Pasadena Independent School District Superintendent Kirk Lewis said.

According to a release from the district, the officer arrived at 12:45 a.m. Tuesday at the campus of Miller Intermediate School in the Houston suburb of Pasadena. The officer called for backup but before help arrived he spotted the boy leaving the building.

Corbett said when the boy came out of the structure, he was backlit and appeared as more of a silhouette to the officer, who was standing under a light. The officer then told him to stop, Corbett said.

"The officer observed he had his hands at his chest and he thought that what was in those hands was a gun," said Corbett, whose agency is investigating the shooting along with the Harris County District Attorney's Office. School district police were investigating the burglary.

The boy, dressed in a black hoodie and black shorts, was standing about 20 feet away on some steps outside the classroom when he was shot in the upper chest, near his right shoulder. Corbett said the boy was not shot while trying to run away but had stopped and was facing the officer when he was hit.

It was not until after the shooting that the officer discovered how old the suspected burglar was and that he was not armed but carrying a mesh backpack, which contained contents from the building, according to authorities.

The district said that the incident was the first time in the history of its police department, which was established in 1981, that a firearm was discharged by an officer as part of a law enforcement action.

The portable classroom where the incident occurred was often used for after-school activities but wasn't currently being used as a classroom. Classes at the school went on as scheduled Tuesday.

"We want to make the day as normal as we can for our students," Lewis said.