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Four teenagers have been arrested on suspicion of arson following a fire at a former council headquarters.

Fire crews were called to Shire Hall on Glanhwfa Road, Llangefni, Anglesey, at 21:20 GMT on Sunday.

The road reopened at about 11:00 GMT on Monday and no injuries were reported.

The blaze was most likely started deliberately, according to a joint investigation by North Wales Police and North Wales Fire and Rescue Service.

The four arrested teenagers are in police custody.

Det Insp Richard Griffith said: “We believe that this fire was caused by deliberate ignition and our investigations are underway to understand the circumstances."

He appealed for anyone in the area at the time to contact the force.

Five fire crews, two aerial ladder platform appliances, an incident command unit and a high volume pump appliance were there at the height of the blaze.

People living nearby were advised to keep doors and windows closed overnight because of the amount of smoke.

The building, which used to be home to Anglesey council and dates back to the late 19th Century, measures about 50m by 30m (165ft x 100ft).

Crews from Llangefni, Holyhead, Bangor and Menai Bridge were initially called to the incident but were replaced by colleagues from other stations.

Mike Plant of North Wales Fire and Rescue Service said crews were called to a "well-developed" and "substantial" fire.

He added: "The fire was surrounded by four o'clock in the morning, we got to the point when the flames were really doused down."

Crews remained at the scene to remove sections of the roof and make sure there were no hotspots before the stop call on Monday morning.