(CNN) -- A bus explosion that killed one and injured dozens in Kenya was caused by a man believed to have ties to a militant group, authorities said Wednesday.
The grenade attack Monday injured at least 40 people in a Uganda-bound bus, Kenyan police spokesman Eric Kiraithe said.
It left blood stains and luggage scattered at the site of the attack in Nairobi's central business district.
Most of the injured were Ugandans headed home for the holidays, the spokesman said.
He said the suspect, a 35-year-old Tanzanian, was the only fatality in the attack. It was unclear whether he had accomplices.
"Police suspected him because he was quite nervous and resisted attempts to have his luggage examined," Kiraithe said.
After an altercation with the police, the suspect dropped his bag, which exploded.
"We have strong reasons to believe there's a link between him and Al-Shabaab," Kiraithe said.
He declined to comment on any evidence linking the suspect to the militant group that has been battling Somalia's transitional government for years. The group, considered the al Qaeda proxy in Somalia, has previously threatened attacks on Uganda and Burundi. The two countries contribute troops to the peacekeeping effort in Somalia.
Kiraithe said it is "logical to believe he targeted the bus because it would have Ugandans."
The bus was bound for Uganda's capital, Kampala. The nation's authorities had warned of holiday terror attacks by the Islamist group hours before the blast.
Kenya and Uganda have boosted security along their borders.
"We are investigating the crime together with Ugandan authorities," the spokesman said. "This is a busy travel period so we're also adding more security and taking all measures to make sure it's safe."
Al-Shabaab has claimed responsibility for July twin bombings in the Ugandan capital that killed more than 70 people watching the World Cup.