Three boys have been arrested in connection with the fire at Willow Place car park, Corby, on Tuesday evening.
Officers arrested the 12, 13 and 14 year-olds on New Year’s Eve in connection with the fire, which is believed to have been started deliberately.
Two teenage boys, aged 14 and 17 and two men, aged 19 and 20, were arrested on Tuesday night while officers attended the scene of the fire.
All seven people arrested in connection with the fire have been released on bail pending further enquiries.
Police would still like to speak to anyone who may have been in the area of Willow Place on Tuesday evening, shortly after 6pm.
Witnesses or anyone with information about the incident are asked to contact police on 101 or Crimestoppers anonymously on 0800 555111.