Four people in East Sussex escaped unhurt after their homes were set on fire as they slept.
A 24-year-old lodger in one of the two houses in Vale Road, Seaford, raised the alarm at 0400 BST on Wednesday after hearing the crackling of flames.
The lodger escaped along with a 21-year-old woman, a 16-year-old boy and their neighbour, a 64-year-old woman.
Police said they believed diesel was used in the arson attack, which caused significant damage to both homes.
The fire melted the tarmac on the driveway and caused the garage roof to collapse. The roof of the second property was also damaged and water damage affected other parts of the home.
'Reckless and dangerous'
Det Con Richard Holder said: "This was without doubt a deliberately ignited fire, set alight while innocent people were sleeping inside their homes.
"This reckless and dangerous act could have ended in tragedy.
"The suspects would have stank of diesel as they used diesel from four fuel cans outside one of the homes to ignite a fire on the driveway.
"They would have left the scene at around 4am and we believe they took the five-gallon fuel cans with them."
Homeowner Shaun Tutt, who was out at the time of the fire, said: "My neighbour was within five minutes of losing her home to the fire, as the direction of the wind blew the flames towards her house.
"She's lucky to be alive.
"My lodger thankfully raised the alarm, making sure everyone, including my neighbour, made it out safely."