POLICE were last night trying to identify a mystery white powder that caused hundreds of people to be evacuated from an office.

The powder was sent in an A4 envelope to the Student Loans Company (SLC) office in Darlington and opened by a worker yesterday.

It was decided to evacuate the building when a member of staff reported feeling ill shortly after the envelope was opened. She was treated at the scene by paramedics and is recovering, police said.

Officers are now trying to discover the source of the envelope and what it contained.

The incident happened before noon in the SLC’s Memphis office at the Lingfield Point Business Park.

Police and other emergency services, including the Hazardous Area Response Team, were called to the scene and a decision was made to evacuate the 650 staff.

Durham Police said the premises were evacuated as a precaution. Staff were moved to a nearby building before being sent home early.

Assistant Chief Constable Michael Banks moved to reassure the public that there was no immediate danger.

He said: “This incident is under control and I must reassure people that we are not aware, at this stage, of any risk to the wider public.”

A force spokeswoman said: “The ongoing police investigation into the source of the envelope and the nature of the powder will continue.”

Last night, Derek Ross, director of operations at Student Finance England, said: “The Darlington site is now safe and staff will return for their next shift as normal.

“We have taken steps to ensure there is minimum disruption to our customer service.

“Our Student Finance England calls are being answered at our other sites.

“Our dedicated EU student phone number is currently unavailable. We are sorry if this incident disrupted our service for customers.”

Eddie Humphries, the estates manager at Lingfield Point, said this was a very unusual incident for the business park.

He said: “I’ve been here for 19 years and we’ve never seen anything like this. We had to evacuate another company once about ten years ago, but nothing like this.

“However, it all seems to have gone very well. The emergency services reacted very quickly and efficiently.

“Our job was to be as helpful as much as we can be and to keep the remaining customers answered as much as we could with the right information to keep the estate running as smoothly as possible.”

Last month, it was revealed that the SLC had to pay back more than £107m since the late Nineties in overpaid loans.

A total of 40,050 graduates overpaid in the tax year ending in April last year, compared with 117 a decade earlier.

Students have complained that it often takes months to repay the money owed to them.