A series of deadly tornadoes and thunderstorms ripped through the southern U.S. this week, leaving hundreds of people dead and thousands injured. President Obama toured "rubble-strewn" neighborhoods in Alabama on Friday, calling the damage "heartbreaking" and "nothing short of catastrophic," and pledging that the federal government would help rebuild the region. Here, a guide to the tornadoes' toll as of 4 p.m. Friday:

At least 164
Tornadoes reported across the South on Wednesday, according to Reuters

At least 318
People killed by the twisters, according to the Associated Press

More than 900
Tornadoes reported in the U.S. since April 1, according to Reuters

Southern states in which the tornadoes reportedly caused fatalities: Alabama, Arkansas, Georgia, Kentucky, Louisiana, Mississippi, Tennessee, and Virginia

People killed in Alabama, according to the Associated Press

People killed in Mississippi

People killed in Tennessee

Unofficial death toll in Tuscaloosa, Ala., according to a city spokeswoman cited by CNN

People injured in Tuscaloosa

Up to 1 million
People in Alabama left without power, according to Reuters

National Guard troops activated to help with storm-related cleanup and rescue efforts in Alabama

People who spent Wednesday in Red Cross shelters across nine states, according to a spokeswoman cited by The New York Times

Peak wind speed, in miles per hour, of a tornado that struck Smithville, Miss., on Wednesday. The National Weather Service rated that storm an EF-5, the highest rating given to tornadoes. The last EF-5 storm in the U.S. hit Iowa on May 25, 2008.

Deaths caused by the Smithville tornado

People killed by a "tornado super outbreak" that hit 13 states on April 3 and 4, 1974, the most recent disaster with a comparable death toll

People killed by the "Tri-State Tornado" of 1925, which struck Missouri, Illinois, and Indiana and remains the deadliest tornado storm in U.S. history

Sources: Associated Press, CNN, The New York Times, Reuters, LiveScience