Houston, Nov 18 (IANS): Three students were in custody after making emergency calls about a fake active shooter at a high school near Houston, the largest city in the US state of Texas, authorities said.

The George Ranch High School, part of the Lamar Consolidated Independent School District, was placed on lockdown as a precaution immediately after the threat on Thursday, Xinhua news agency quoted the authorities as saying.

A lockdown means that all of the outside and inside doors to campus were locked.

A nearby middle school and junior high school were also placed under secure status, where the outside doors are locked and the inside operates as usual.

It took 30 minutes for law enforcement agents on the scene to determine there was no active or credible threat, according to Heather Patterson, principal of the George Ranch High School.

Fort Bend County Sheriff Eric Fagan said more students may be detained and could face charges of "swatting and terroristic threats".

"Doing things like this, it takes away a lot of resources from people who possibly need it," Sheriff Fagan said.

"I called out my SWAT, and several officers came out... I'm not complaining about that. What I'm complaining about is the false call."