The migrants were reportedly of Afghan origin, the police added, while informing that they were in good health with some provided first aid.
The finding on Monday was made after officers carried out a routine check on the truck, which was travelling on a section of motorway near the town of Xanthi.
The discovery came after 39 people, all believed to be Vietnamese nationals, were found dead in a refrigerated truck in Britain last month, highlighting the risks of illegal migrant routes to Europe, even for those avoiding perilous travel by sea.
The truck's refrigeration system was switched off, but some of the group complained of breathing problems and were taken to a hospital. The driver, a man from Georgia, was arrested. Two people in the United Kingdom and eight in Vietnam have been charged over the deaths.
Airbnb to ban 'party houses' after a deadly Halloween shooting
The sheriff's department said they were responding to a noise complaint at the house around the time the shooting was reported. Three people, all from the Bay Area, died at the scene and a fourth died at the hospital, authorities initially said.
Road accidents involving migrants have become more common in northern Greece in recent years.
Police have arrested dozens of people believed to be involved in human trafficking so far in 2019.
Scores arrive day by day, overwhelming camps on Greek islands dealing with Turkey the place over 34,000 persons are staying, majority in deplorable dwelling circumstances.