Nairobi, Kenya (CNN) -- A Kenyan man pleaded guilty Wednesday to being a member of the Islamic militant group Al-Shabaab and was charged with causing grievous bodily harm in a grenade attack on a crowded bus station Monday.
Police said they found an arms cache with the man, identified as Elgiva Bwire Oliacha, also known as Mohamed Seif.
Kenyan government spokesman Alfred Mutua said authorities were tracking him even before the attacks.
Earlier Wednesday, Mutua said twin grenade attacks in Nairobi earlier this week were not the work of Al-Shabaab.
"It was two young guys inspired by Al-Shabaab," he said, "but not directed by Al-Shabaab leadership."
Mutua said authorities had arrested one suspect and that they hope to arrest the second soon.
One person was killed and 20 injured in the Monday blasts -- one at a nightclub and a second at a crowded downtown bus station during rush hour.
The explosions came two days after the U.S. Embassy in Kenya warned it had credible information regarding an imminent terror attack. The embassy offered no details on who might carry out such an attack, but it said it had limited U.S. government visits and urged citizens to consider deferring travel to Kenya.
The attack was likely to target places where foreigners congregate, the embassy said.
Kenya has been on edge since it sent troops across the border into Somalia to pursue militants with Al-Shabaab, an Islamist group that the United States and other countries consider a terrorist organization. Kenya sent troops into Somalia after the recent abductions of tourists and aid workers in Kenya. It blames the abductions on Al-Shabaab, which has denied involvement.
Al-Shabaab has threatened to attack Kenya if it does not withdraw its forces from Somalia.
CNN's Christabelle Fombu and Jack Maddox contributed to this report.