It could take several days to determine the cause of a four-alarm blaze that gutted the abandoned Angle Lake Elementary School in SeaTac late Saturday, a King County Fire spokesman said Sunday.

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It could take several days to determine the cause of a four-alarm blaze that gutted a boarded-up school in SeaTac late Saturday, a King County Fire spokesman said Sunday.

Firefighters believe no one was inside the abandoned Angle Lake Elementary at the time of the blaze, but they can’t say for sure because they retreated outside after fighting the fire for only 10 minutes. Nine South King County fire agencies had responded to the blaze around 5 p.m. Saturday.

“The situation was deemed unsafe because of the integrity of the building and how fast the fire was spreading,” said Dave Nelson, spokesman for South King County Fire Zone 3.

Concern the structure could collapse delayed the start of an investigation, he said, adding that the Port of Seattle this week will supply a crane that allows crews to hover above the building and inspect the damage. In the meantime, Port police are guarding the building to prevent anyone from entering.

“The fire was so hot, and it was a masonry building on all sides, so there’s a risk of it collapsing,” Nelson said.

The fire, which took more than an hour to knock down, destroyed about half of the old brick building and left a burned-out shell.

The 1928 structure is owned by the Port of Seattle, which bought it about two years ago for unspecified future airport projects, Port spokesman Perry Cooper said.

Surrounded by a chain-link fence and with plywood covering the windows, the building is listed on the State Register of Historic Places, said Cyndi Upthegrove, Highline Historical Society board member.

The Port, however, had set in motion plans to tear the building down, deeming it beyond rehabilitation. One of the original seven schools of the Highline School District, the building also served as SeaTac’s first City Hall. It ceased to be a school in 1975.

The King County sheriff’s arson unit and Port of Seattle Police are investigating the fire.