Here are three primary sources that were referenced in today’s “Starting Point with Soledad O’Brien” -

* Our top story was the devastation caused by tornadoes in the Midwest. Tornado strength can be measured on the “F-scale,” which is named after T. Theodore Fujita. For more information on the F-scale, visit this website from the National Oceanic and Atmospheric Administration.

* Christine Romans interviewed The Edge, U2’s longtime guitar player. The Edge is taking part in a new MTV documentary that addresses the issue of homeless young people. “The Break” is hosted by Anne Mahlum, who founded the non-profit Back on My Feet program, which also assists the homeless. For more information about The Break, including the trailer, visit MTV’s webpage for the show.

* Nada Bakri, the widow of New York Times reporter Anthony Shadid who died last month while on assignment in Syria, talked with Soledad live this morning about her husband’s legacy. If you want to learn more about the two-time Pulitzer Prize winner, check out one of his recent articles from The Times. Also, this YouTube video may be Shadid’s last interview before he died.