NAIROBI, Kenya (AP) — Gunmen killed two policemen guarding a church, snatched their rifles and then opened fire on the congregation from inside and out today, killing 15 people and wounding 40, security officials said.

Two gunmen entered the simple wooden church in the city of Garissa at around 10:15 am, while two others waited outside, police commander Philip Ndolo said. When the congregation fled the attack inside, they ran straight into another hail of bullets from gunmen outside, he said. At least one grenade was detonated in the attack.

Overturned wooden benches littered the church afterward. A victim wearing a simple blue dress lay on the sandy earth outside. 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, a churchgoer. "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 lie down. Within no time we had gunshots all over. Everybody was shouting and wailing in pain."

The bloodiest of the two attacks came against the African Inland Church in Garissa, a city some 195 kilometres (120 miles) west of the Somali border. Ndolo said 15 people were killed and at least 40 wounded. A grenade attack against a second church in Garissa wounded three people.

Garissa Mayor Ismail Garat called the church 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 told reporters he wanted an investigation carried out before assigning blame to the group many people 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 two policemen guarding the church, shot them at point-blank range and took their rifles. The official spoke only on condition he wasn't identified because he is not allowed to speak to media.

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 "vile" and "disgraceful" attacks and said they showed the necessity of defending the rights of Christians to celebrate their faith and "oppose irresponsible acts that fuel hatred among religions."

Such a heinous attack could be a copycat strategy from Boko Haram, the group of Islamist militants in Nigeria that has made gruesome, deadly attacks against Christian churches one of their hallmarks.

Garissa is one of two major Kenyan towns near the border with Somalia. It lies just to the west of the Dadaab refugee camp, which houses nearly 500,000 Somali refugees. On Friday armed attackers kidnapped four international workers with the Norwegian Refugee Council and are believed to have taken them over the border into Somalia.

A top security official suggested after that assault that the attackers came from within the camp. Kenyan officials have long complained Dadaab and its inhabitants are a threat to Kenya's security. Kenyan officials hope to see the Dadaab refugees move back to Somalia, but they cannot force the refugees to move without breaking international law and courting wide international condemnation.

Areas of northern and eastern Kenya along the border with Somalia have suffered a series of gunfire and grenade attacks over the last year. Militants attacked a church in Garissa in December, killing two people.

Kenya sent troops into Somalia last October to hunt al-Shabab fighters. The militants, who are allied with al-Qaida, have threatened repeatedly to carry out revenge attacks for Kenya's push into Somalia. Sunday's attacks appear to be part of that trend.