Image This advisory sent by a police official alerted security officials about a threat to churches from a radical Islamist group, National Thowheeth Jama’ath. Government officials have blamed the group for the attack.

Local ties with such groups may have been strengthened in recent years by Sri Lankan Muslims who traveled to fight in wars in Syria and Iraq, said Sameer Patil, a national security fellow at Gateway House, a foreign policy research group in Mumbai, India. With the Islamic State having recently lost its last patch of territory in Syria, he said, the group’s foreign fighters are now more likely to return home to Sri Lanka and other countries.

“It was just a matter of time before that would hit them on their own soil,” Mr. Patil said.

Scott Stewart, the vice president of tactical analysis at Stratfor, a geopolitical consulting firm based in Austin, Tex., noted that the attackers were unusually successful for a group with no track record of large-scale assaults.

The initial evidence showed that all seven suicide vests detonated. That is certain to worry law enforcement agencies. Initial efforts by small, homegrown extremist groups are usually marked by some degree of failure. Some of the bombs fail to detonate entirely. Others explode early or late, and still others cause smaller blasts than their builders intended.

Whoever designed the suicide vests used in the Easter Sunday blasts showed considerable expertise, he said, and photographs indicate that the bombmaker had access to a lot of military-grade high explosives.

But Joshua A. Geltzer, who was senior director for counterterrorism in the Obama administration and is now the executive director of the Institute for Constitutional Advocacy and Protection at Georgetown University, said that it was now much easier for relatively unknown to groups to be successful on their first try.