KUMAMOTO, Japan (Kyodo) -- A woman in her 70s, who according to local officials had been sleeping in her car following Tuesday's powerful earthquake in Kumamoto Prefecture, southwestern Japan, died of suspected heatstroke, the prefecture said Sunday.

Her death could be the first disaster-related fatality following the magnitude 7.1 quake, the prefecture said.

The woman's vehicle had run out of gasoline when she was found Thursday.

The death toll has now reached 38, including the woman and two other cases still under investigation as to whether they were caused directly by the earthquake, according to the prefectural government. The number of people in serious condition stands at 13, updated from 11 Saturday.

The toll includes nine dead at Nippon Paper Industries Co.'s plant in Yatsushiro and seven at an Aeon shopping mall in Kashima that was rocked by an explosion following the temblor.

Meanwhile, Aeon Co. said Sunday that one male customer who was found unconscious at the company's shopping mall in Uki on Friday after its ceiling collapsed had died at a hospital.

More than 8,000 remain in evacuation shelters or other temporary accommodations.

Japanese Prime Minister Sanae Takaichi is scheduled to visit Kumamoto Prefecture on Monday to grasp the needs of those affected by the earthquake and reflect them to the government's support measures.

The quake registered the maximum level of 7 on Japan's seismic intensity scale in Uki and Hikawa at 4:27 p.m. on Tuesday. At that level, people cannot remain standing or move without crawling and may be thrown through the air, according to the Japan Meteorological Agency.

The municipal government of Uki said a woman in her 70s, who was evacuated to a shelter, was diagnosed with COVID-19, adding that she will be hospitalized to receive medication before being moved to a special shelter for those who need care.