Kenyan police say they have arrested four people in connection with deadly grenade attacks in Nairobi blamed on the Somali militant group al-Shabab.

Officials said Monday they had arrested Slyvester Opiyo and three teenagers and brought them in for questioning. Opiyo was previously arrested in December on suspicion of having links to al-Shabab.

Officials accuse attackers of hurling grenades at the outdoor terminal of a major bus station in the capital on Saturday evening. The attack killed at least six people and injured about 60.

This is the latest in a string of gun, bomb and grenade attacks that have killed dozens of people since Kenya sent troops into Somalia in October to fight al-Shabab.

The group recently formalized an alliance with al-Qaida, and East African governments have declared the militants to be a regional threat.

Al-Shabab has been trying to overthrow Somalia's government and turn the country into a strict Islamic state.

The group claimed responsibility for bombings in Uganda's capital that killed more than 70 people in July 2010.

No one has claimed responsibility for Saturday's attack.