Overturned wooden benches littered the church afterward. Witnesses reported seeing the four gunmen flee in dark blue outfits and masks.
"We were deep in prayers preparing to give our offerings," said a visibly shaken David Mwange. "We first had a loud bang from outside which we mistook to be coming from the rooftops. We then had gun shots which made us to lie down. Within no time we had gunshots all over. Everybody was shouting and wailing in pain."
A grenade attack against a second church in Garissa wounded three people.
Garissa Mayor Ismail Garat called the assault "evil."
"We are not used to witnessing such kinds of acts in our country, where people are just shot in broad daylight. We really want to know who the heartless people who did this are," he said.
Ndolo said he wanted an investigation before assigning blame to the group many in this region assume is at fault: al-Shabab, the most dangerous militant group in Somalia.
Another security official said two attackers walked up to the police guarding the church, shot them at point-blank range, and took their rifles. The official spoke only on condition he was not identified because he is not allowed to speak to reporters.
The police were guarding the church because of the increasingly dangerous security situation near the border with Somalia and because Somalia's Islamist militants have made Christian churches a common target.
The Vatican spokesman condemned the " attacks and said they showed the necessity of defending the rights of Christians to celebrate their faith.
The White House also condemned the attacks, saying: "At a time of transition, peace and stability are essential to Kenya's progress."