The mistakes occurred over 13 years and began after a state Supreme Court ruling ordered the Department of Corrections to apply "good time" credits earned in county jail to state sentences, Gov. Jay Inslee said.
About 3% of all releases were given excessive "good time" credits. The median number of days that inmates were mistakenly released early is 49 days, officials said.
Correction officials are now trying to locate the ex-offenders who were released too early, and state authorities will ensure the ex-offenders will "fulfill their sentences as required by law," officials said.
Those former inmates, however, will be given "day for day" good credit for their time in the community. Depending on how much time remains to be served on their sentence, offenders will go to work release or back to prison, officials said.
That may amount to a relatively small number of ex-convicts who need to be brought back, said Jaime Smith, spokeswoman for the governor's office.
"So far Department of Corrections has identified seven offenders that need to be brought back in, and we've brought in five," she said Tuesday.
Corrections officials learned of the problem in 2012 and initiated a fix, but "for reasons that will be investigated, the sequencing fix was repeatedly delayed," the governor said.
A newly hired chief information officer brought new attention to the problem, Inslee said.
"That this problem was allowed to continue for 13 years is deeply disappointing to me, totally unacceptable and, frankly, maddening," Inslee said in a statement.
The governor has hired two retired federal prosecutors, Robert Westinghouse and Carl Blackstone, to conduct an independent review of how the error occurred and why it took 13 years to resolve.
"These were serious errors with serious implications. When I learned of this, I ordered DOC to fix this, fix it fast, and fix it right," the governor said in a statement.
"I have a lot of questions about how and why this happened, and I understand that members of the public will have those same questions. I expect the external investigation will bring the transparency and accountability we need to make sure this issue is resolved," he added.
Inslee ordered a temporary halt to releasing any state prison inmate "until a hand calculation is done to ensure the offender is being released on the correct date," officials said.
A software fix to calculating release dates should be in place by January 7, officials said.