"Cables might have caught fire," Tokyo University professor Nato Sekimura said. "It's understandable that people evacuated to study what happened in the area to secure their safety."
Meanwhile, traces of radiation from the damaged nuclear plant were still detected miles from the plant, including in some vegetables and water supplies, raising alarm by Japanese residents and spurring U.S. officials to continue urging precautionary steps.
The State Department distributed potassium iodide tablets to U.S. government personnel and their families in Japan "out of an abundance of caution" but instructed them not to consume the pills yet. Potassium iodide helps the body block absorption of some radioactive materials.
The Pentagon also ordered the aircraft carrier USS George Washington and destroyer USS Lassen to move farther out to sea off the coast of Japan out of radiation concerns.
ABC News' Luis Martinez, Kirit Radia and Barbara Garcia and the Associated Press contributed to this report.