GAS canisters were found in a car that ploughed into South Oxfordshire District Council's offices before a huge blaze ripped through the premises, say police.
The two-storey building in Crowmarsh Gifford was targeted in an arson attack, along with two other properties in the early hours of this morning (Thursday). Officers believe the incidents are linked.
Thirteen crews from Berkshire, Oxfordshire, Buckinghamshire and Wiltshire were called to tackle the blaze on the first floor of the council building at 3.20am.
A bomb disposal team were at the scene inspecting the site.
Thames Valley Police say the incident is "not thought" to be linked to any terrorist activity.
Firefighters were also called to a blaze at a funeral parlour in nearby Benson Lane and a third at a thatched property at Quakers Corner, Rokemarsh, near Benson, where they were alerted by the occupier. An 80-year-old women managed to escape from the property safely and is helping officers.
Gas canisters were also found at these locations, Superintendent Andy Boyd, area commander for South Oxfordshire and the Vale of White Horse told a press conference this afternoon.
Officers are currently searching another address in Rokemarsh, believed to be the home of a 47-year-old local man who has been arrested on suspicion of arson and is currently in custody. Gas canisters had also been found at this address, he said.
All the fires were now under control and crews are dampening down.
Council leader John Cotton said: "I'd like to express my thanks to the Oxfordshire fire service. It looks like their fast actions have saved a large part of the building and we're very grateful for that."
He said the authority had contingency plans for this kind of event and these had been set in motion.
But he added: "As you can see the planning department has pretty much disappeared."
A "skeleton staff" were now working from the council's offices in Abingdon, he added.
A statement from Thames Valley Police said: "At this stage the police investigation is focusing on all three fires being linked and are believed to be suspicious. It is believed that a vehicle collided with the SODC building prior to the fire.
"There are cordons in place at all three locations and residents and motorists are asked to avoid these areas.
"Nobody has been reported injured at this time as a result of these fires, however following these fires the structures of the buildings affected are being examined for safety.
"This is being investigated as a criminal act and is not thought to be linked to any terrorist activity."
Taxi driver Raleigh Hullock, 50, had just dropped off his fare in Crowmarsh Gifford at about 7.30am.
He told the Henley Standard: "It looked as if a vehicle had been driven into the reception area. That's either been set alight or caught alight and sent the whole building up."
Mr Hullock, from Wallingford, managed to get to within 50 yards of the building and said he spotted a "burnt-out shell of a vehicle".
He added: "The aerial platform was up and the jets trained on the building to knock the fire back. The first floor was totally alight."
Tash Altay, of Jethro Tull Gardens, Crowmarsh Gifford, saw firefighters tackling the blaze at about 6am. He said: ?Smoke was billowing away, going in the opposite direction to the houses in Howbery Farm or they would have been evacuated.
?We were 30 or 40 metres away from the building and you could see the blaze even though it was dark. The top floor on the north end was totally in flames.
?You don?t expect something like this in sleepy Crowmarsh Gifford. The police weren?t really saying anything so we could tell it was serious.?
Ann and David Beasley have lived in neighbouring Winters Field for more than 40 years and were woken up by the blaze.
Mrs Beasley said: ?We could see flames shooting up over the reception building and I said to David that it looked really bad.
?The noise was awful, it was like a fireworks display. You could hear glass shattering constantly and there were two big explosions, it was horrific.
?The flames crept along so quickly, there?s no way they could have saved the building. Water was gushing down but it wasn?t doing anything.
?I was really scared and shaken up. I rang my son and just burst into tears down the phone.
?I?ve never seen anything like this. We?ve already had nine phone calls from family and friends to see if we are OK. It?s very scary and upsetting when it happens on your doorstep.?
David Etheridge, Oxfordshire?s chief fire officer, said: ?We are in the process of dealing with three major fires in South Oxfordshire, which occurred within a very short time-frame."
About 30 people have been evacuated from their homes in Rokemarsh and the British Red Cross are helping them at Berrick Salome and Roke Village Halls.
A spokesman for the district council said: ?We are putting our business continuity plan into place to make sure the effect on the most important services is kept to a minimum. We will set up an emergency phone line for people who have urgent enquiries.?
The council says services affected by the fire include planning and building control, environmental health, finance and housing.
Services running as normal include waste collections and street cleaning, leisure centres, housing benefits payments and council tax payments.
It hopes to have its online services restored by the start of next week.
District councillor Will Hall, who also sits on Henley Town Council, wrote on Twitter: "The damage is terrible, the crime is awful but there's also something deeply sad about seeing our bit of democracy in South Oxon hurt."
Henley town clerk Mike Kennedy wrote: "Hope this despicable attack against democracy is severely punished. My heart goes out to all staff & members. The town council will support SODC any way we can, including use of buildings as a satellite office if needed."
The council offices house about 400 staff, including those in the housing, planning, building control and leisure departments.