99% Invisible did a great episode on squirrels, covering their history in major American cities (they were introduced, not native) and the havoc the wreak on our infrastructure
They were later also brought in to cities in other parts of North America, and there it was introducing a non-native species. Worse, in many of those areas there were species of native squirrels living outside the cities. After the Eastern Gray's were brought to the cities, they often spread to the forests outside the cities, and took over the habitat of the native squirrels.
Here in the state of Washington, there is only one small area known to still have a native Western Gray squirrel population. The Eastern Grays, introduced a little under 100 years ago, have taken their place everywhere else.
I have no idea why whoever wanted squirrels in Seattle decided to bring Eastern Grays from across the country, instead of simply going and trapping a few of the then abundant native Western Grays outside the city, and using those.