Two days after heavily-armed gunmen shot up a concert hall just outside Moscow, some people who had family and friends at the venue still don't know the fate of their loved ones.

More than 130 people were killed at the Crocus City Hall concert venue on Friday night. More than 140 people were also reported injured.

Rescuers continued Sunday to comb through the damaged building, looking for victims and possible additional survivors.

The attack has been claimed by an affiliate of the Islamic State terror group, ISIS-K, but Russian President Vladimir Putin has tried to tie Ukraine to the slaughter.

Russia National Day of Mourning

Russia observed a day of national mourning on Sunday. Mourners laid flowers outside the concert hall, according to the Associated Press.

Electronic billboards across the country displayed the image of a burning candle and the word "Skorbim" which means "we mourn," the BBC reported.

Many events across the country were canceled according to state news agency RIA Novosti.

Russia's Federal Security Service (FSB) reports the incident has been labeled as a "terrorist attack."

Armed terrorists wielding assault rifles unleashed gunfire in the building's lobby before proceeding to the main concert hall, where a band was set to perform.

Igor Pogadaev has not heard from his wife since she went to the concert. She sent him photos from the concert and then went silent.

He rushed to the concert hall after he saw reports of the attack.

"I went around, searched, I asked everyone, I showed photographs," Pogadaev told the Associated Press. "No one saw anything, no one could say anything."

Russia says it has arrested 11 people in connection with the attack, including the four suspected gunmen.