Four men in their 40s are still being questioned by police after a home-made rocket was found in a car stopped in Londonderry on Thursday night.
The vehicle was stopped at Westway in Creggan. Three of the men were in the vehicle, the other was arrested nearby.
A senior police officer has said he has no doubt the rocket was to be used to try and kill his officers.
Chief Supt Stephen Martin said the device was of a type used extensively by insurgents in Iraq and Afghanistan.
"This a weapon which is used primarily to attack armoured vehicles and kill the occupants of those vehicles," Chief Supt Martin said.
"Unfortunately it has been used extensively in places such as Iraq and Afghanistan to deadly effect.
"So we are obviously concerned that there was one of these in the city last night, but delighted that we have been able to take it out of circulation before it was used for its deadly purpose."
Chief Supt Martin said three of the rockets, which he termed "explosively formed projectiles" had previously been seized in Northern Ireland recently, but this was the first in Derry.
"It would be my firm belief that this would have been used against police within the city here," he said.
"Clearly it is a worrying development, this is a device which is designed for one purpose and that's to kill."
He said the arrest operation had been "proactive" and was targeting dissident republican activity.
Some residents had to leave their homes while the Army dealt with the device.