Amid an intense heatwave, three million people were left without power.
Power companies are warning that some may not have electricity restored for up to a week.
Officials have warned that the heatwave - compounded by the loss of air conditioning due to power outages - could threaten the very young, old and sick.
In Bradley county, eastern Tennessee, the high temperature has been blamed for the deaths of two brothers, age three and five, who were playing outside in 105F (40.6C) heat, Reuters news agency reports.
‘Dangerous’
Maryland, West Virginia, Virginia, Ohio and the District of Columbia have all now declared states of emergency.
Virginia Gov Bob McDonnell said the state - where six people died from the storms - had had its largest non-hurricane power outage in history.
“This is a very dangerous situation,” he said, according to Associated Press. More
