Three men, including a serving policeman, have been arrested over a hoax call to police over a terror plot to kidnap an officer.
Detectives are questioning the men - aged 25, 28 and 31 years old and all from Birmingham - on suspicion of conspiracy to pervert the course of justice.
The "false and malicious" call to West Midlands Police on December 8 last year led to a full-scale heightening of security measures.
Officers were called in for special overnight briefings and advised not to come to work or go off shift in uniform.
Bosses also reminded them about basic police station security measures.
It also prompted Scotland Yard to make its own assessment of the information received by West Midlands Police.
A West Midlands Police spokesman said: "At a time when the national threat level was severe, the threat was considered credible and police acted swiftly to protect officers and police staff.
"A subsequent investigation revealed that the call was false and malicious."
The 28-year-old police officer, who works at Birmingham West & Central Local Policing Unit, was also arrested on suspicion of misconduct in a public office and misuse of police systems.
Meanwhile, the other two suspects have also been arrested on suspicion of conspiracy to commit misconduct in a public office.
A 31-year-old man arrested on the day of the offence was eliminated from inquiries and has since been deported by Border Officials.
In October last year the threat level to police officers across the country was raised to substantial, meaning a targeted attack is a strong possibility.
The national security level has remained at severe since August last year, meaning a terrorist attack is highly likely.