All police stations in the Sri Lankan capital of Colombo have been advised to be on high alert for a truck and van suspected to be carrying explosives, a Colombo Police spokesperson told CNN.
Akshat Saraf was vacationing with his wife and young child over the Easter holidays in Sri Lanka. Having enjoyed a restful holiday, the family overslept on Sunday morning in the room on the 25th floor of the Shangri-La Hotel in Colombo.
A few seconds later they heard another boom. Looking out the window, he noticed a group of bystanders turn and start pointing at the building.
"That's when I realized probably the sound, the explosive noise, did come from our building and I asked my wife to pick up the passports and we immediately left the room.," said Saraf.
Footage provided to CNN shows a man identified as a suspect by Sri Lanka state TV SLRC walking around St. Sebastian's church on Easter Sunday morning, shortly before it was rocked by an explosion.
The blast at the crowded service in Negombo was one in a series of coordinated attacks -- which targeted churches and luxury hotels within minutes of each other -- across the country.
What you can see: The suspect can be seen carrying a large backpack as he makes his way across the courtyard towards where the busy service is already underway.
At one point, the alleged bomber can be seen briefly stopping to pat a child on the head before continuing on his way.
Making his way through the throngs of people perched outside the service, he enters a side door of St. Sebastian's, close to the altar.
The next frame, according to police but not provided publicly, shows the suspect detonating his bomb.
Sri Lanka police said they would not comment on the video until the conclusion of the investigation.
Islamic extremists carried out Sunday's coordinated attacks in retaliation for a March attack on two mosques in New Zealand, said Ruwan Wijewardana, the Sri Lankan State Defense Minister.
Addressing parliament today, Wijewardana said that initial investigations showed "the chain of bombings" was carried out by "a radical Islam group" which he named as the National Tawheed Jamath (NTJ).
No group has yet claimed responsibility.
Last month's mosque attack in Christchurch, New Zealand, saw at least 50 people murdered in a shooting rampage by a white supremacist. The rampage began at lunchtime on a Friday, when mosques were full of worshippers.
Sri Lanka Police tell CNN the death toll has now risen to 321 in the Easter Sunday bombing attacks.
CNN's Will Ripley is at St. Anthony's Shrine in Colombo, the scene of one of Sunday's bombings. It remains an active crime scene and the interior is so badly damaged that investigators haven't been able to examine it all yet.
Ripley spoke to Fr. Jude Fernando, the administrator of the holy site, who said that when the blast went off around 8.45 a.m., he was in his office and just about to walk into the church.
"I came inside the church and I saw people screaming, crying and I started, with the help of the community, and staff and priest, to send each and every one to the hospital," he said.
He said the church was packed because it was Easter Sunday -- around 1,000 devotees were in attendance, he estimated.
Fernando appealed to Sri Lankan Christians to "please be calm and quiet and pray, because our God is not a god of revenge, he's a god of love, he's a god of peace... let's follow our master and spread the good news.
"Continue to pray for one another, and don't do any harmful acts," he said.
Watch the video here.
South Korea joined the US, Australia and China today in cautioning its citizens about travel to Sri Lanka.
The country's Ministry of Foreign Affairs raised the Sri Lanka travel-alert level to "highly cautious."
"We ask our citizens who are residing in Sri Lanka to take extra caution and our citizens who plans to travel the region to carefully consider the need to travel," it said.
From CNN’s Jake Kwon in Seoul
Global headlines about the attacks will have likely delighted the perpetrators, said Anne Speckhard, director of the International Center for the Study of Violent Extremism, who also noted that the coordinated explosions fit the "ISIS and Al Qaeda playbook."
"They want to make a big press presence, a big statement, here’s what we’re capable of doing, here’s how much we hate you, they even like the backlash, they think it unites Muslims to their cause," she told CNN.
Speckhard expressed concern that the attacks could inspire copycat actions.
Funerals at St. Sebastian’s Church were suspended today, after services for 16 victims had already taken place. A priest at the church in Negombo, a city to the north of the capital Colombo, told CNN's Ivan Watson that a funeral procession had been planned, but was stopped. No official explanation has been given.
Bishop Maxwell Silva told Watson that the attacks had changed the dynamic for Sri Lanka's Christian community and that they "have to talk precautions now."
He said that new security measures could include developing "our own security system"—for example, "we can ask people not to bring their purses to church."
"At this time we are speechless. We never had any communal violence," another church official, Father Cyril Gamini Fernando, told CNN.
He said that because police had asked not to have large crowds of people, "I feel there must be a threat," and added that he did feel like a target.
Most of the crowds have left the church grounds now, where the funerals had been taking place.
Inside the damaged building, CNN staff could see police personnel in medical scrubs and surgical masks. There are still police lines still around the church compound.