The Electra Fire was 40% contained on Thursday after burning more than 6.5 square miles.

Wednesday's humidity and calmer winds helped the fight against the blaze, but more than 1,200 structures were still threatened on Thursday, the San Francisco Chronicle confirmed. One first responder has been injured.

The fire began Monday afternoon in the Sierra Nevada's foothills along the Amador-Calaveras county line and forced evacuations in both counties. The fire trapped 100 people at a Pacific Gas and Electric powerhouse on Monday as the wildfire blocked the only exit. All were safely evacuated.

(MORE: Wildfire Traps 100 People in Power Station During July 4th Festivities)

Evacuations in Amador and Calaveras counties were still in place, though residents along Clinton Road and Butte Mountain Road to were permitted to return to their homes late Wednesday.

According to The Associated Press, officials are investigating whether the fire was caused by fireworks or a barbeque during July 4th celebrations.

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