Nairobi, Kenya (CNN) -- The death toll has risen to six following an attack on a Kenyan bus station that left dozens wounded, the Red Cross said Sunday.
At least 68 people are undergoing treatment at a Nairobi hospital, according to the agency.
The explosions, which authorities believe are grenade attacks, occurred Saturday at a normally-crowded bus station in central Nairobi.
"We still need to complete investigations and we cannot immediately confirm who did this, but we highly suspect that it is Al-Shabaab or sympathizers of Al-Shabaab," said Charles Owino, the police deputy spokesman. "Eyewitnesses say that grenades were thrown from a vehicle and the suspected perpetrators fled from the scene."
There have been several instances of grenade attacks at bus stations and at a bar in Nairobi in recent months.
One person was killed and 20 injured in October when an attacker hurled grenades into a bar and a bus station. In that attack, a Kenyan man pleaded guilty to being a member of the Islamic militant group Al-Shabaab and was charged with causing grievous bodily harm.
The East African nation has been on edge since it sent its troops into Somalia to pursue Al-Shabaab militants after the abductions of tourists and aid workers in Kenya. It blames the abductions on Al-Shabaab, which has denied involvement.
Al-Shabaab, which the U.S. considers a terror group, has threatened to attack Kenya if it does not withdraw its forces from Somalia.