In San Luis Potosí at least four people died due to heat stroke and six more deaths are under investigation, according to local health services.

Ten cities in Mexico have registered record-high temperatures, including the capital, authorities said on Friday, amid a searing heat wave that has prompted blackouts nationwide and pushed the power grid to the brink.

In the normally temperate high-altitude capital of Mexico City, North America’s largest metropolis, thermometers on Thursday peaked at 34.3 degrees Celsius (93.7 degrees Fahrenheit), a tenth of a degree higher than the record hit just a month earlier.

Neighboring Puebla broke its previous record of 34.3 C — set in 1947 — when it reached 35.2 C on Thursday.

In San Luis Potosí, at least four people died due to heat stroke and six more deaths are under investigation, according to the San Luis Potosí Health Services. More than 40 people were hospitalized due to heat stroke-related symptoms.