A fire broke out early yesterday in hawkers' stalls in Fa Yuen Street, Mong Kok.
Principal fireman Tsang Wai-ming said the fire was suspicious as two rows of stalls had ignited at the same time.
The case was passed on to police for further investigation. Government laboratory technicians were conducting tests.
The fire broke out about 5am, destroying more than 50 stalls and slightly injuring six people including a fire fighter. Two men, three women and the firefighter were treated at Kwong Wah Hospital.
The fire started in stalls at the junction of Fa Yuen Street and Prince Edward Road West and was categorised as No 3 alarm fire seven minutes later.
It spread quickly because many of the stalls were stocked with highly combustible clothes, shoes and other goods.