The blaze caused around £2m worth of damage on 29 August after a row of 48 lorry trailers were set alight in Woodston, to the south of the city.
Three 16-year-old boys and a 19-year-old man have since been released pending investigation. The fifth, a 15-year-old boy, has been released on bail.
Cambridgeshire Constabulary confirmed in a statement that inquiries were ongoing.
No one was injured during the course of the incident, which sent black, acrid smoke billowing over the East Anglian city.
Witnesses reported hearing car alarms blaring and “multiple explosions”– with fire services later confirming the bangs were caused by trailer tyres.
Amy Price, 45, who witnessed the blaze, said: “As soon as I got outside I could see the thick acrid smoke with ash falling nearby ... the smell was just disgusting. There were continuous streams of ambulances, fire appliances, investigation units and incident support units in attendance.
Emergency services were first called to the site at 7pm, where 55 firefighters from across the region tackled the inferno for the next six hours.
Images from the scene showed the vehicles hollowed out and surrounded by burnt out washing machine drums and and spare parts for fridges.
A spokesperson for Whirlpool, which owns Hotpoint, told the Peterborough Telegraph they were “deeply concerned” to hear the incident may have been caused by arson.
They added the company was “assisting the police in every way we can as they continue their investigations".