Kenya and Uganda are tightening security at borders, airports, and bus stations after a grenade blast Monday struck a Uganda-bound bus and killed three people at a main bus station in downtown Nairobi.

No group has claimed responsibility for the blast, but Al Shabab, the Somali insurgent group with ties with Al Qaeda, has carried out attacks on Uganda for its support of the Somali transitional government that Al Shabab militants are fighting.

“We believe there's a connection between the threats we're getting from Al Shabab and other Al Qaeda-affiliated groups and the attack in Nairobi," Uganda's top police officer, Inspector General Kale Kayihura, told reporters Tuesday.

Kenya has been reluctant to send peacekeepers to join the Ugandan-led African Union Mission in Somalia (AMISOM), but it provides strong diplomatic support for the Somali transitional government and it works closely with other East African nations in trying to rein in jihadist Islamic militias, such as Al Shabab.

Kenya’s President Mwai Kibaki vowed to arrest anyone associated with the apparent suspect in Monday's blast, named by Kenyan police as Albert Molanda, a Tanzanian national who had carried a Russian-made grenade in his luggage. Mr. Molanda was said by witnesses to have dropped his bag, when he saw that security guards were scanning luggage to be loaded on the bus. The grenade inside the bag then exploded, killing Molanda and two other bystanders.

This is the second time in six months that Uganda has been the target of a bomb blast. In July, suicide bombers hit two restaurants in Kampala, Uganda, that were full of local people and tourists watching a World Cup game. Al Shabab took credit for the bombings, which killed 79 people, and promised more attacks.

"In these incidents which have happened, we have found that most people perpetrating this type of crime are people who have recently converted to Islamic faith,” Kenyan police commissioner Matthew Iteere told reporter in Nairobi on Tuesday. “These are the people who have been radicalized and indoctrinated."

Kenya’s Security Minister George Saitoti told the Monitor that Kenya might call in the FBI to help with the investigation into the blast, and to trace the grenade to its source. Mr. Saitoti also called on police to be more vigilant at all entry points and at key installations such as bus stations and airports.

Traveler George Kiwanuka was standing outside the station at the time of the bombing.

Mr. Kiwanuka is quite doubtful that the Kenyan police will be able to ramp up security in the way that they have pledged. “They need scanners, sniffer dogs, and bomb detectors, which is not possible,” he says.

Sheila Mutasa, another traveler bound for Uganda, says that this attack happened because of Uganda’s presence in Somalia. “I think the Ugandan Army should withdraw from the AMISOM's peacekeeping mission,” she says. “[Ugandan President Yoweri] Museveni's surprise visit in Somalia could have triggered all this.”