By Roland Buerk BBC correspondent in Dhaka

Frustration is widespread at the lack of prosecutions

Saturday's grenade attacks that killed at least 18 were the latest in a series of explosions that have left more than 140 dead in the past five years.

There have been few clues as to who is responsible, but some fear the country is facing a concerted campaign of violence.

"If you look at the type of activities, then you find similarities, as if people having the same kind of ideas and objectives are doing this," says Altaus Samad, a widely respected political commentator and the editor of the weekly paper, Ekhon.

"If I was an investigator looking at this chain of events, then I'd certainly try to investigate the fanatical Islamic groups."

Most deadly

The Awami League, at whose rally Saturday's attack was targeted, blames the government and has called for opposition groups to unite against religious extremism.

"This government believes in fanatical terrorist politics, this government should go," says Awami League leader Sheikh Hasina, who narrowly survived on Saturday.

There are a lot of factors we have to examine. We don't rule out anybody Shahudul Haque, police inspector general

The attack was the third explosion at an Awami League meeting this year and the most deadly.

"At present we are keeping our options open because we just cannot blame anybody without any evidence," say Shahudul Haque, inspector general of Bangladesh's police and the most senior officer in the land.

And it is not just the Awami League that has been targeted.

Earlier this year there were two attacks at the Hazrat Shahjalal shrine in the northern city of Sylhet.

The first was during a festival in January involving dancing, which some view as un-Islamic.

Then bombers struck again in May, killing three people and injuring British High Commissioner Anwar Choudhury.

It was Mr Choudhury's first visit as Britain's representative to the district from which his family had emigrated to the United Kingdom.

Other blasts have rocked cinemas in Sylhet, Mymensingh and Satkhira.

Coalition

The government has rejected opposition charges that religious extremists could be to blame.

Cinema bombings in Mymensingh in December 2002 killed 17

"One thing I want to make sure - this country has been established and identified as a moderate Islamic country," says Harris Chowdhury, political secretary to Prime Minister Khaleda Zia.

"There is no room for fundamentalism here. If fundamentalism appears on the scene that should be dealt with seriously."

In all but one of the bombings over the past five years, the police have failed to make any arrests, leading the opposition to allege that those responsible have support in parts of the government.

Bangladesh is ruled by a four-party coalition that includes two Islamic parties.

"I think not only me, any person with my responsibility would feel a bit frustrated because things are happening," says Shahudul Haque, the police chief.

"As a police officer I feel it is my duty to find out who is doing those things. Of course, it is frustrating. Of course."