Police were at Stormont five minutes after they were called about Michael Stone's attack on Parliament Buildings, the Criminal Justice Minister has revealed.
But it took 16 minutes for the call to be made, according to David Hanson.
Mr Hanson said the loyalist began his attempted assault at 10.55am on November 24, while MLAs were in session.
Security staff wrestled Stone into submission at the entrance to Parliament Buildings and a call to police was placed at 11.11am.
A PSNI unit arrived at 11.16am, 21 minutes after Stone began his attack.
As the Assembly conducts a security review in the aftermath of the November 24 incident, questions remain about the fact that armed and uniformed police guards were withdrawn from Parliament Buildings in 2002.
Unarmed security staff wrestled Stone to the ground, preventing him from attacking MLAs or members of the public.
At least eight armed police bodyguards - who protect the DUP MPs sitting in the Assembly - were reported to be in the building at the time Stone attempted to gain access with pipe bombs and other weapons.
But uniformed police guards had not been present for years.
In January 2002, the PSNI told Assembly chiefs that uniformed officers would be withdrawn on days the Assembly was not sitting. However, later that year uniformed guards also stopped attending on sitting days.
Since the attack, a uniformed police presence has returned to Parliament Buildings for the Assembly's recent sittings.
Mr Hanson's account said the Stone security alert began at 10.55 am.
"Security staff in Parliament Buildings made an emergency call at 11.11 am to the PSNI Operational System requesting assistance," he said.
"The PSNI arrived at Parliament Buildings at 11.16 am, five minutes after the emergency call was placed.
"I pay tribute to the response of all staff at the Parliament Buildings following the alert."