Five tourists from China and four Thais, including the two pilots, were on the Cessna Caravan C208B aircraft that went down 100 km (62 miles) southeast of Bangkok, 11 minutes after losing contact with ground control following take-off.
All aboard the charter plane are presumed dead, said Chonlatee Yongtrong, the governor of the Chachoengsao province, the site of the crash, as authorities scramble to investigate the cause.
"We found many human remains," the governor told reporters late on Thursday, adding that the muddy terrain complicated the task of searchers.
"The plane dropped vertically, so we have to dig 10 m (33 ft) into the ground."
Photographs of the site show aircraft debris scattered over a forested, boggy area, while rescue workers dig with hoes and use a pump to extract water from some areas, while police forensic units seek to recover and reassemble the bodies.
The plane, operating flight TFT209 headed for the eastern province of Trat, had taken off from the Suvarnabhumi airport in the capital on Thursday afternoon.
Registered to Thai Flying Service Co Ltd, according to the aviation regulator, the craft lost contact with ground control in Bangkok 11 minutes after take-off, provincial officials said.
