A 30-year-old man has been arrested on suspicion of murder after a fatal house fire, police said on Saturday evening.
Hertfordshire Police said a man believed to be in his 60s was pronounced dead at the scene of the blaze in Westerdale, Hemel Hempstead, early today.
Emergency crews were alerted to the fire at 12.06am and scenes of crime officers were working with specialist fire brigade investigators to determine the cause.
Detective Chief Inspector Mark Ross, leading the inquiry, said: "Whilst the investigation is still in its early stages, we are treating the man's death as suspicious at this time.
"We are providing specialist support to the victim's family at this distressing time."
The officer added: "Although this incident occurred during the night, the property is in a residential area and there may have been people who may have seen or heard suspicious activity.
"I would appeal to anyone who believes they can assist the investigation to contact us as soon as possible."
Anyone with information should contact the Bedfordshire, Cambridgeshire and Hertfordshire Joint Major Crime Unit via the non-emergency 101 number.