Herts Police have confirmed they are treating the arson attack in Lower High Street, between 4.15pm and 4.20pm on Sunday (April 19), as “religiously aggravated”.
However, they say it is believed to be “isolated” and not linked to recent offences in London.
Detective Superintendent Mark Clawson said: “Firstly, I would like to make it clear that we do not tolerate hate crime in our communities in Hertfordshire. We are working hard to find the person(s) responsible.
“I am keen to hear from any witnesses or anyone with information. This includes those who might have mobile phone footage of the incident or offender(s).”
He added: “In particular, I am especially keen to trace a group of young males who were seen in the area around the time of the incident.
“If you were one of these people, or think you know who they may be, please get in touch.”
Information can also be shared online, or by calling 101, quoting the reference ISR 481 of 19 April.
It comes after people linked to the Watford area were arrested on suspicion of arson attacks at a synagogue and the offices of a Persian-language news organisation in northwest London last week.