A grenade was lobbed into a mosque in the southern Philippines before dawn on Wednesday, killing two Muslim religious leaders, in the second attack in days on a place of worship in the restive south, officials said.

The attack, in the city of Zamboanga, occurred three days after a bombing at a cathedral on the nearby island of Jolo, which killed 20 people outright. More than 100 people were wounded in that attack, one of whom died on Tuesday, bringing the death toll to 21, reports nytimes.com.

The police said it was too soon to identify a motive for the Wednesday mosque attack. They said the possibility that it was retaliation for the Jolo bombing could not be ruled out, but they also cautioned against speculation. The southern Philippines, which unlike the rest of the mostly Roman Catholic country has a Muslim majority, has long been plagued by insurgencies.

The Islamic State has claimed responsibility for the Jolo attack, which it said was carried out by suicide bombers.

Some security officials initially blamed Abu Sayyaf, a local Islamist group, and said the two blasts at the Cathedral of Our Lady of Mount Carmel had probably been set off remotely, using a cellphone. Suicide attacks are rare in the Philippines.

But on Tuesday night, President Rodrigo Duterte contradicted that version of events. He said he had been briefed by intelligence officials who told him the attackers were suicide bombers.

‘That is really terrorism, and that is really suicide,’ Mr. Duterte said. He said that a woman wearing a crucifix had detonated a bomb inside the cathedral, and that a male companion had blown himself up outside minutes later.

The Philippine military, by contrast, has said that bystanders saw a woman, presumably the female bomber, put a bag onto a pew and leave the cathedral shortly before the blasts.

Rommel Banlaoi, the head of the Philippine Institute for Peace, Violence and Terrorism Research, said the attacks in Jolo and Zamboanga suggested that a law granting autonomy to Muslims in the south — which voters approved last week throughout the southern part of the Mindanao island group, except in a small province dominated by Jolo — would not quickly lead to peace.