Shortly after the crash on Saturday, government officials said all those on board were feared dead.
The Antonov plane, owned by local company Equatair, was on an internal flight between the capital Malabo on Bioko island and the city of Bata on the mainland.
"The plane crashed near Baney, 17km from Malabo," said Ricardo Mangue Obama, a deputy prime minister in charge of the government task force investigating the crash.
"We don't know the cause of the accident. We can't confirm any death toll but clearly it was a serious accident and we have no information on any survivors so far," he said.
Mangue Obama said the 55 on board included six crew members. Local media reports said as many as 80 people could have been on the plane but there was no confirmation of that.
The plane left Malabo around 1000 GMT on Saturday but disappeared off the radar screen shortly afterwards.