Two bloody attacks in Dagestan, in southern Russia, on Sunday ignited fears of extremist violence on the home front, as the Kremlin pours resources and bodies into its sprawling war in Ukraine.

Gunmen slaughtered at least 20 people and set fire to houses of worship, and video quickly circulated on social media of men with rifles standing in a street and shooting, including at passing vehicles. Though little else is known about the attacks, they touched a nerve in a region long strained by separatist and ethnic tensions.

Here is what we know:

What happened, and who is responsible?

Groups of gunmen launched seemingly coordinated attacks on synagogues and Orthodox churches in two cities — Makhachkala, Dagestan’s capital, and Derbent — that are more than 70 miles apart.

Though Russian officials called the violence acts of terrorism, they did not blame the attacks on any specific people or groups. No organization has claimed responsibility, and the motive remains unknown.

Russia’s Investigative Committee opened a terrorism investigation.

Before a deadly assault in March at a concert hall outside Moscow, U.S. intelligence agencies warned of a pending attack by an offshoot of the Islamic State, and after that attack, they quickly said the group was responsible.

But on Monday, American officials said they still had not made an assessment about who had committed the shootings in Dagestan.