How does Amulek know how the people would be destroyed? Alma 10:22

How does Amulek know how the people would be destroyed? Alma 10:22

Amulek is prophesying to the people. He declares that if it weren't for the prayers of the righteous, that everybody in Ammonihah would be destroyed, just like in the days of Noah - except rather than the destruction coming by flood, it would come by the sword, famine, and disease. If the people would repent and not kick out the prophets, including Alma, they would be spared. Otherwise, they would be destroyed.

Spoiler alert, the people really didn't like to hear that. His words provoked them to burn all the believers in a mass killing, perhaps also Amulek's family. It is really horrifying to think about that.

Answer:
Amulek was visited by an angel in Alma's house. I bet the angel told him that the people would be destroyed. It seems likely it was the main message Alma and Amulek were sent to share with the people of Ammonihah. It kinda fits the pattern of prophesies.