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Gunman report sparks Nasa alert
TV grab of Building 44 at Nasa's Johnson Space Center in Houston, Texas
The alert comes after this week's massacre in Virginia
The US space agency, Nasa, has evacuated a building at the Johnson Space Center in Houston after reports that a gunman had been seen.
A Nasa spokesperson could not confirm reports that shots had been fired - or that a man had barricaded himself in.
Police were called to the scene, an engineering office known as Building 44, at about 1340 (1840 GMT).
A Nasa contractor, Jacobs Engineering, has told the BBC that one of its employees is involved in the incident.
The company said it was co-operating with police.
US media have been reporting that the suspect is a white man in his early 50s, but Nasa could not confirm this.
School secured
The Houston police department told the BBC they had sent a helicopter, a canine unit and a special weapons team to the space centre.
Building 44 houses communications equipment and engineering laboratories.
The Johnson Space Center contains Nasa's mission control which oversees the agency's space flights.
The centre's staff, numbering several thousand across the sprawling complex, were initially told to remain in their buildings but after several hours were allowed to leave the complex.
An intermediate school near the building was also secured for several hours.
The alert comes less than a week after a gunman killed 32 students and teaching staff at Virginia Tech university before killing himself.
The BBC's Adam Brookes in Washington says there has been a rash of security alerts across the US, which is also marking the eighth anniversary of the Columbine school massacre in which 15 people died.
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