A power outage was impacting a whole lot of people in San Mateo Thursday morning, and its cause is still a mystery.

Around 36,000 PG&E customers were without power Thursday morning in and around San Mateo with an outage that started around 8:26 am. As NBC Bay Area reports, two hours later, with power restored for some, 28,400 customers were still in the dark.

As of 2:15 pm Thursday, according to PG&E's outage map, it appeared that thousands were still without power along the 280 corridor and to the east of it, in Burlingame and San Mateo.

The orange areas shown below indicate between 500 and 5000 customers without power.

Map via PG&E

Crews were working to get power restored by Thursday afternoon, according to a statement from PG&E.

The utility said that, according to a preliminary assessment, the cause of the outage was an equipment problem.