A blaze which ripped through a housing development in Burnley is under investigation by police.
Flames swept through several homes on a new housing estate in Sycamore Avenue.
Firefighters battled the blaze in the timber-framed buildings for more than an hour on Friday.
Fire chiefs are treating it as suspicious and said those responsible had risked their lives.
Watch manager Steve Harrison, of Burnley Fire Station, said: “Building sites are dangerous places anyway, but if someone has broken in and set it on fire - the way the buildings are made and the potential way they accessed through the window, they have put themselves in serious danger.
“The fire burned quickly and it could have easily caught them and they would not stand a chance of escaping it.
“They have risked their lives.”
The blaze broke out at 12-17pm on Friday afternoon on the new housing development.
Firefighters were forced to break into the secured site as flames took hold of the property.
Watch manger Harrison said: “When we arrived it was well alight. Fortunately it was spotted early. Fire in those unfinished timber frame buildings has the potential to spread really rapidly. It was a number of houses linked together in a cross shape - they are quite large buildings - it would have created havoc.
Once they are going they are difficult to stop.
“Two lads went inside to fight the fire. We were there for an hour and a half making sure the fire was out.”
An investigation has been launched into the blaze and the police were collecting evidence and witness statements near the scene.
Fire crews have been trying to contact the owners of the housing development.
The fire caused significant damage to the roof and corner of the property.