Those who have survived a devastating tornado in Joplin, Missouri, should do their utmost to live up to the example set by those who died while helping others escape the storm, President Barack Obama said Sunday at a memorial service for the victims.

Obama spoke about two of the heroes from the twister, which barreled into Joplin packing 200-mph winds a week ago Sunday.

One of them, Dean Wells, directed his co-workers and customers at Home Depot to safety, returning again and again for more people until a wall of the store fell on top of him, the president said.

And Christopher Lucas, 26, a manager at a Pizza Hut, herded employees and customers into a walk-in freezer, finding a bungee cord to hold the door shut from the inside and wrapping the other end around his arm. Lucas held on as long as he could, Obama told the crowd Sunday, "until he was pulled away by the incredible force of the storm. He died saving more than a dozen people in that freezer.

"There are heroes around us all the time," the president said. "And so, in the wake of this tragedy, let us live up to their example, to make each day count, to live with a sense of mutual regard, to live with that same compassion that they demonstrated in their final hours. We are called by them to do everything we can to be worthy of this chance we've been given to carry on."

The tornado swept a 13-mile path, the National Weather Service said Sunday, raising its earlier estimate of six miles.