Trooper Julie Judson said it's unclear how many of the cows have died.
She said the tractor-trailer was heading eastbound on I-90 Saturday morning when its double-decker container full of about 30 cows tipped over on its side near south Seattle.
She said crews have righted the container, with the cows inside. The container has been moved to the shoulder, and arrangements are being made to move the animals.
The incident caused some traffic delays.
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