Kenya's Church leaders have blamed the government for a grenade attack at a rally on Sunday that led to six deaths.
The explosions at a Nairobi prayer meeting campaigning against a draft constitution caused a deadly stampede.
After an emergency security meeting, Prime Minister Raila Odinga confirmed it was a grenade attack and said a top police team was investigating it.
All but a handful of ministers in the shaky coalition government are backing a "Yes" vote to the draft charter.
"Having been informed over and over that the passage of the new constitution during the referendum is a government project, we are left in no doubt that the government, either directly or indirectly, had a hand in this attack," said a statement signed by the National Council of Churches of Kenya and 14 other churches and groups.
"Who else in this country holds explosive devices?" they asked.
But Mr Odinga called for an end to speculation about who is responsible until investigations are concluded.
Many Kenyans doubt the Church leaders' claim that the government could be behind the blasts, especially as it seems most people are already backing the "Yes" campaign, says the BBC's Will Ross in Nairobi.
The rally was organised by Christian groups opposed to a draft constitution because it retains recognition of existing Islamic courts and includes a clause on abortion.
Kenyans are due to vote on the new constitution in a referendum in August.
As part of a power-sharing deal to end deadly riots following elections in December 2007, it was agreed that a new constitution would be written.
Both supporters of Mr Odinga and President Mwai Kibaki generally support the draft constitution.
The document provides for greater checks on presidential powers and more regional devolution.
However, Christian Church leaders are campaigning for a "No" vote after an amendment to abolish abortion on medical grounds failed.
Supporters of the new constitution deny that it opens the door to legalised abortions.
Christian Church leaders also oppose the continued recognition in the draft document of Islamic family courts.
The Islamic Kadhi courts - set up under British colonial rule - mainly deal with matters of marriage and inheritance for Kenya's Muslim minority.