At least six people, including four Saudi nationals, were arrested on Sunday over an explosion that killed two people at a church in northern Tanzania on the same day.
Tanzanian President Jakaya Kikwete called the blast in the town of Arusha an "act of terrorism."
"This is an act of terrorism perpetrated by a cruel person or group who are enemies of the country," Kikwete said in a statement on Monday.
No group has claimed responsibility for the explosion, which also left as many as 60 people wounded. Militant attacks are very rare in Tanzania, and the attack on the Catholic church marks one of the country's first, unlike in neighboring Kenya and Somalia, where attacks are common.
According to witnesses, Sunday's explosion was caused by a bomb or grenade thrown into a crowd near the entrance of the newly opened St. Joseph's Roman Catholic Church as it was about to hold its inaugural Mass, which was attended by the Pope's envoy to Tanzania.
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Arusha's commissioner Magesa Mulongo confirmed that the four Saudis arrested arrived at the Arusha airport on Saturday. Tensions between Tanzania's Christian and Muslim communities have been high in recent months, but the Tanzanians arrested were confirmed to be Christians.
"Investigations are ongoing," Mulongo added.
Reports say Tanzania has seen a rise in violence between Christians and Muslims in the past year. Because there are no official records of religious affiliation in the country, it is not clear which group is the majority in Tanzania.