At least 20 people died and 15 others were injured Saturday when a bus overturned on the Pan-American Highway in southwestern Colombia, police said.

The bus was traveling between the port city of Tumaco, in the southwestern corner of Colombia, and Cali, 200 miles (320 kilometers) to the northeast when the incident occurred.

"Unfortunately, we have a toll of 20 people dead," said Captain Albertland Agudelo of the Narino department traffic police.

"The hypothesis being investigated is that it appeared to suffer a mechanical failure," Agudelo said.