She was joined by Secretary of Homeland Security Janet A. Napolitano, who arrived in the city of about 56,000 to survey the damage and supervise federal disaster relief workers already on the ground.
President Obama will visit Moore over the weekend.
The tornado, described by one official as “the storm of storms,” claimed 24 lives and injured more than 230 others. Property damage is estimated to be between $1.5 billion and $2 billion.
Residents were allowed to return to their homes for the first time Wednesday, with local officials confident there are no more victims buried beneath rubble.
Clean-up efforts will continue throughout the weekend, just as the first victims’ funerals begin.
Those funerals will include services for several of the seven children killed when Monday’s tornado tore through their school.