"This was an incredibly nasty incident and the staff were very shaken but fortunately unharmed, which is our number one priority.
"We will be offering them any assistance and any support they require after this terrible incident.
"We are also working with the police and hopefully witnesses will come forward to help catch these people."
More top news
A gang of fraudsters who swindled elderly victims out of thousands of pounds have been jailed for a total of more than 10 years.
Three of the UK's rarest birds of prey - which nest in East Anglia - have been fitted with satellite tracking devices.
Hundreds of probation jobs are under threat across the region after Sodexo announced cuts at its community rehabilitation centres.