Many of the dead were found alone, in homes without air conditioning or fans. Some were elderly — one as old as 97. The body of an immigrant farm laborer was found in an Oregon nursery.

As forecasters warned of a record-breaking heat wave in the Pacific Northwest and western Canada last weekend, officials set up cooling centers, distributed water to the homeless, and took other steps. Still, hundreds of people are believed to have died from Friday to Tuesday.

An excessive heat warning remained in effect for parts of the interior Northwest and western Canada on Thursday.

The death toll in Oregon alone reached 79, the Oregon state medical examiner said Thursday, with most occurring in Multnomah County, which encompasses Portland.

In Canada, British Columbia’s chief coroner, Lisa Lapointe, said her office received reports of at least 486 “sudden and unexpected deaths” between Friday and Wednesday afternoon. Normally, she said about 165 people would die in the province over a five-day period.

She said it was too soon to say with certainty how many deaths were heat-related, but that it was likely the heat was behind most of them.

Washington state authorities have linked more than 20 deaths to the heat, but authorities said that number was likely to rise.

In Oregon’s Multnomah County, the average victim’s age was 67 and the oldest was 97, according to county Health Officer Jennifer Vines.

In a telephone interview on Thursday, Vines said she had been worried about fatalities amid the weather forecasts. Authorities tried to prepare as best they could, turning nine air-conditioned county libraries into cooling centers.

Between Friday and Monday, 7,600 people cooled off amid the stacks of books. Others went to three more cooling centers. Nearly 60 teams sought out homeless people, offering water and electrolytes.

But the efforts weren’t enough, she said: “It’s been really sobering to see these initial (fatality) numbers come out.”

This week’s heat wave was caused by what meteorologists described as a dome of high pressure over the Northwest and worsened by human-caused climate change, which is making such extreme weather events more likely and more intense.

Seattle, Portland, and many other cities broke all-time heat records, with temperatures in some places reaching above 115 degrees Fahrenheit (46 Celsius).