(CNN) -- Kenyan police detained scores after gunmen masked in balaclavas hurled grenades at two churches this week, authorities said Wednesday, killing 17 people in the latest attacks in the nation.
Authorities held more than 30 people after the twin explosions in Garissa town Sunday, according to Philip Ndolo, the regional deputy police chief.
Of those detained, at least 20 remain in custody after police released others because their interrogations yielded no results, he said.
The arrests include Kenyans and foreigners of Somali origin, according to the police chief.
The explosions near the Somali border wounded at least 40 others when gunmen stormed the Catholic Church and African Inland Church when services were under way, prompting a stampede.
No one immediately claimed responsibility.
"We are sure al-Shabaab is behind the attacks, but right now we are only speculating," Ndolo said.
Attacks have escalated since Kenyan forces invaded neighboring Somalia last year to battle Islamist militant group al-Shabaab, blaming it for kidnappings of foreigners in the nation.
When Kenya pursued the terror group, it threatened retaliatory attacks, saying it considers the incursion an affront to Somalia's sovereignty.
Since the invasion in October, Kenya has been hit by a series of grenade attacks blamed on the militants or their supporters.
The grenade attacks have mostly hit the capital of Nairobi, the port city of Mombasa and northeastern towns and refugee camps near the border with Somalia.
Al-Shabaab is linked to al Qaeda and has been designated a terrorist organization by the United States.
U.S. officials have issued warnings of potential attacks in the country since the incursion.