A threat to kidnap and murder a serving West Midlands police officer led to a security alert being issued to staff, the BBC understands.
The force has confirmed counter-terror officers are investigating an “anonymous but credible” tip-off.
Staff were first told on Monday night after the force received the call.
Assistant Chief Constable Garry Forsyth said the force was taking the threat “extremely seriously”, but there was no increased risk to the public.
Officers have been urged to take extra precautions, including not wearing uniform during journeys to and from work.
West Midlands Police said it had “implemented additional security measures” after obtaining the information, but the force declined to confirm the exact nature of the threat.
A spokeswoman said counter-terror officers were continuing to assess the anonymous information the force had received.
Officers were held on duty while they were given the security reminder as they started their shifts.
The advice included guidance about travelling to and from work in partial uniform and police station security.
The BBC's West Midlands correspondent Phil Mackie said security had been stepped up at stations across the region.
A group of men were jailed in 2008 over a plot to kidnap and behead a soldier in Birmingham.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said his force had not issued guidance to officers, but was analysing the threat.
“Once we've assessed it, we'll decide whether or not to give any further advice,” he said.
The force tries not to “overreact” in situations where a threat has been made, he added.
The Home Office said it was aware of the incident and remained “in touch” with the force.
“All police staff were issued with guidance from the national policing lead in October reminding them of the need to remain alert to any possible dangers,” said a representative.
Mr Forsyth said the information received related “to the safety of West Midlands Police officers and police staff”.
“We have taken the opportunity to remind all employees of the need to be vigilant. Our priority remains serving our communities and protecting them from harm,” he said.
“Officers remain on patrol and our staff continue to respond to calls for service as usual.”
In October, the threat level to police officers across Britain was raised to substantial.
The national security level remains at severe, signifying a terrorist attack is highly likely.