Hundreds of Kenyan youth who have trained with Al Qaeda-linked militants in neighboring Somalia have returned home and now pose a major security threat to this East African nation, a top police official said Friday.

Police Commissioner Mathew Iteere said that a Kenyan man trained by al-Shabab militants in Somalia hurled a grenade into a church last Sunday, killing two people and wounding 15.

Iteere released a photo of a man known only by the alias Amar. He said witnesses at the church were able to identify Amar as the person who threw the grenade Sunday. After the grenade exploded some church members attempted to pursue Amar but the suspect fired a gun at his pursuers, Iteere said.

Some church members who survived the blast said they suspected the attack was the result of a land dispute. However, Iteere said an al-Shabab leader -- Sheik Ismael Ali -- claimed responsibility for the attack and vowed to dispatch suicide bombers.

Police spokesman Eric Kiraithe said that Amar is Kenyan but is not a member of Kenya's large Somali community. In a similar case, another non-Somali Kenyan, Elgiva Bwire Oliacha, 28, was sentenced to life in prison last October after he pleaded guilty to charges related to throwing a grenade at a bus stop in Nairobi that same month.

A July U.N. report warned that al-Shabab was recruiting non-Somali members from countries in East Africa and was giving rise to a new generation of East African jihadists. The fighters represent a new security challenge for the region and wider international community, the report said.

The report, by a panel of U.N. experts monitoring arms embargoes against Somalia and Eritrea, said that in the past al-Shabab's presence in Kenya was concentrated primarily within the ethnic Somali community.

Many of those who are returning to Kenya after training with al-Shabab are from a slum in Nairobi, from a border region near Somalia or from the Kenyan coast, Iteere said.

Sunday's attack was the most recent of a series of grenade and gunfire attacks since Kenya sent troops to Somalia in October.

In March, grenade explosions at one of the main bus stations in Kenya's capital killed nine people and wounded 40, the deadliest in the series of attacks. Police say at least 40 Kenyan civilians have died from the attacks.

Al-Shabab militants have vowed to carry out a major attack in retaliation for Kenya's deployment of troops into Somalia to pursue al-Shabab.