The three additional deaths came after Houston, the fourth-largest US city, was hit Thursday by heavy rain and winds up to 100 miles (160 kilometers) per hour, leaving downtown streets covered in glass from blown-out windows.
Downed trees and power lines littered residential areas and the National Weather Service confirmed a tornado touched down in the suburb of Cypress.
According to Harris County Sheriff Ed Gonzalez, the new victims included an 85-year-old woman who died after her mobile home was struck by lightning and caught fire.
A 60-year-old man was found unresponsive and pronounced dead after going to his truck to try and power his oxygen tank, after electricity was cut across wide swaths of the storm-hit area.
And a 57-year-old man collapsed and died after trying to move a downed power pole.
Houston Mayor John Whitmire said Thursday after the storm that four people died, though specifics were not immediately available.
Schools in Houston, home to 2.3 million people with an economy centered around oil and petrochemicals, were closed on Friday and non-essential workers urged to stay home.
