COLOMBO (WASHINGTON POST) - As evidence emerges that Islamist militant groups planned and carried out the Easter Sunday (April 21) bombings that killed more than 350 people in Sri Lanka, the country's nervous Muslim communities are scrambling to show that they share the nation's shock and grief.

Mosque leaders have stopped broadcasting prayer calls over loudspeakers to avoid offending mourners. They have put up banners with messages of condolence for the victims. They have met Catholic Church and police officials, and packed food kits for funeral volunteers.

They also are bracing for a wave of anger and possibly violent retaliation that has begun to emerge. Hateful online messages blaming Muslims for the attacks have evaded the government's emergency social media ban, and stones have been thrown at several Muslim homes and businesses.

"We don't know who is behind these attacks, and we condemn them with all our hearts, but many people are saying they were the work of Muslims, so we don't know what could happen next," said Mr Mohammad Jinnah, a jeweller and member of the interfaith harmony committee at the Grand Mosque in South Negombo, 20km from the capital.

"This used to be such a peaceful place, but now everything has turned upside down."

In Muslim neighbourhoods in Negombo, where more than 100 people died in the deadliest of the bombings, dozens of shops have been closed since Sunday, except for those conducting early morning food sales, and fresh produce can be seen wilting behind locked metal screens. In Colombo, Muslims have formed ad hoc neighbourhood watch committees, checking for unfamiliar vehicles and visitors.

And at hundreds of mosques across the country, plans for weekly prayers this Friday are uncertain; religious leaders said they expect attendance to be considerably lower than usual. Some have asked police to guard the gates and have posted signs announcing that all bags will be searched.

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The fear, leaders said, is not so much of attacks by Christians, but from others who might take advantage of the turmoil to cause trouble. In addition, Muslim communities are now being searched by intelligence police with newly expanded powers to conduct raids and make arrests, adding to a general sense of unease.

Security officials have arrested more than 40 suspects in the Easter bombings, all Sri Lankan Muslims. Some are said to be part of a local extremist group against which one mosque in the capital has held protests. Two of the suicide bombers were the sons of a prominent local Muslim spice merchant. The Islamic State (ISIS) also asserted responsibility for the attacks.

Muslims first came to Sri Lanka in the seventh century, when it was the Kingdom of Ceylon, but they remain among the smallest groups in the multifaith society, with a populace of about two million comprising about 10 per cent of the national population. Some have achieved economic success as traders and professionals, but many have remained poor and are employed in low-skill jobs.

Relations between minority Muslims and majority ethnic Sinhalese Buddhists, the nation's dominant group, have been marked by outward peace but simmering tensions for decades, leading to two violent riots, in 2014 and 2018. One source of animosity is the noticeable surge in the number of mosques, some of which have been described as preaching a strict brand of Sunni Islam that was once rare in this relaxed tropical society.

Many Muslims interviewed this week said they had friendly relations with members of other religious groups and were proud of living in a diverse society, but that they did not mingle much socially outside their mosque-centred communities.

Leaders of one mosque in Negombo said they offered joint funeral rites for Fatima Alsa, a teenage Muslim girl from a partly Christian family who died in the bombing at St Sebastian's Catholic Church and was later buried under a palm tree in the local Islamic cemetery.

Later, though, mosque officials said they were politely rebuffed when they made further overtures to local Christians and were urged by church officials to stay at home and avoid any problems.

"We wanted to do more, but we were asked not to," said one mosque member, Mr Mohammad Sufaree, who also expressed indignation that a second meeting with local church officials had been abruptly broken off. "This is why the Muslim community is hesitant to come out beyond its boundaries. We always take risks, but we feel left out in the middle of the jungle."

In Colombo, outside the gold-domed Grand Mosque, a large banner had been strung up, condemning the "ghastly" Easter attacks. A mosque member and community leader, Mr Mohammad Ifam, said that in recent months, the mosque had organised several protests after Friday prayers against National Thowheeth Jama'ath, an extremist group that has been linked to the bombings.

"We condemn the attacks 100 per cent, 1,000 per cent. We put out white flags to show how we felt, and yet they have brought a black name to Muslims," Mr Ifam said.

"This is the first time world terrorism has come to Sri Lanka, and we demand that government take action against the Thowheeth very soon. They must stop it now."

The dominant emotion among Sri Lankan Muslims this week, though, was a quieter mixture of suppressed sorrow at a tragedy they could not publicly mourn - and of anxious anticipation of a punitive backlash they had not earned.

"What these barbarians did has disturbed everything," said Mr Shafi Mula, the manager of one mosque in the capital that is under police guard. "The Christians have always been brotherly with us, but some other people may want to take revenge, or take advantage, especially in rural areas where people are not protected. So we fear," he said. "We fear."