Authorities in Puerto Rico said they were working to restore electricity after an outage struck the island during maintenance on the electricity grid, leaving about 350,000 customers without power during a heat wave.

The outage, which began late Wednesday, struck after Luma Energy, which transmits and distributes electricity in Puerto Rico, said it was carrying out planned maintenance work on power plants around the island.

The company said at around 9:40 p.m. that they were investigating the outage, which it said was linked to disturbances in transmission lines and expected to last throughout the night.

An excessive heat watch was active for many of the areas affected by the outage, including San Juan, the capital, as the National Weather Service warned of the risk of heat stroke. Customers lost power in San Juan, the metropolitan area of Carguas and some areas in the northeast of Puerto Rico, Luma Energy said early Thursday.

Luma Energy said on social media that it was working with Genera PR, which oversees Puerto Rico’s electricity generation, and local officials to restore service in a process expected to continue throughout the night.

Electricity problems have plagued Puerto Rico for years, including after Hurricane Maria devastated the power grid in 2017. The island has struggled to recover since.

Gov. Pedro R. Pierluisi condemned the power companies’ “lack of sense of urgency” in addressing the problems with the island’s aging power grid that he said had been going on for weeks.