The company attacked when a man went on the rampage at the Castlemead building in Bristol said it did not believe the man arrested was a former member of staff.
Equiniti, which has an administration team on level ten of the tower block, confirmed its office suffered “superficial” damage.
Workers at 250ft tall office block had to run for cover when a crazed man hurled fire extinguishers, computers and chairs through a 10th floor window.
The man is understood to have set the fire alarm off at the 17-storey Castlemead building in Bristol and made his way up as staff were evacuated.
He then ran amok, wrecking computers and smashing doors and windows before hurling office equipment including desks and a printer into the street below.
Most of the debris landed on the canopy of the reception area but several people and a number of firefighters were injured.
Police arrested a 49-year-old man at the scene on suspicion of burglary and common assault after he was restrained by a fireman and a Castlemead security guard.
He was released on bail pending further investigation.
A statement from Equinti said: “We can confirm that we are aware of the security incident that occurred on Tuesday night.
“No Equiniti staff were injured as a result of this incident and no staff were present on the floor when the intruder entered Level ten.
“Our office was secure and alarmed and we were immediately alerted to the intruder entering our office via our alarmed door sensor.
“Based on the information that has been made available to us, we do not believe this is a current or former employee of Equiniti Group.
“Equiniti Group treats incidents of this nature very seriously. We immediately deployed our business continuity plan and managed the incident via agreed procedures set out in this plan.
“Internal damage in our office was superficial. Disruption was minimal and we were able to resume operations early the next morning.”
The company added it was working closely with the building’s property management team which is leading the investigation from a property and security perspective.
The incident happened at around 7.20pm on Tuesday evening. A worker, who did not want to be named, said: “Everybody who comes in has to flash their pass to get through a turnstile, but apparently a guy managed to breach the security on the front desk.
“The first thing anyone knew about it was the fire alarm going off, so everybody exited. Everyone got outside to the fire assembly point out the front, and then a fire extinguisher came flying out through the window.
“There was a lot of glass and over the next few minutes a computer, a printer, a desk and a chair came out through the same window of the 10th floor.
“It hit the canopy at the front of the building, and some of it was right outside the front of the building. In the car park next door someone’s car also got hit by a fire extinguisher.
“There were quite a few people in the building at the time, and some of them got hit by a few bits of debris. Someone got injured, but I am not sure how badly.
“Apparently someone was letting off the fire extinguishers all over the place, and when he threw them out they were still going off and spinning all over the place as they fell.
“I have got no idea who it is but there is general speculation that he is an ex-employee of one of the offices in the building.”
A spokesperson for Avon and Somerset Police said: “Shortly before 8pm on Tuesday November 22, police were called to Castlemead in Bristol.
“A man was reported to be throwing items including a fire extinguisher from the tenth floor of the building. Considerable damage had been caused to the inside of the building.
“A 49-year-old man has been arrested on suspicion of burglary and has been released on bail.”
A separate investigation will be held to see whether charges of assault can be brought in relation to injuries suffered by firefighters.