Yes, that would be illegal. An employer owes wages for all time worked, even time spent "training," where the employer directs that training. It sounds to me like you have a wage claim, but whether and how it is worth pursuing are strategic questions that you can decide after consulting with an attorney.
I agree with attorney Thal's answer. You should also consider contacting the Department of Labor & Industries, which is free. They are available on the web at www.lni.wa.gov, then click on "workplace rights" and follow the links to file a complaint or contact a representative. They may be able to investigate and pursue the matter on your behalf at no cost to you. Good luck.
This does not constitute legal advice. You should consult an attorney who can assess the facts of your case and discuss your legal rights and obligations.
Both prior answers are excellent. L&I would likely open your file in this matter and would pressure to get you the exact amount owed and that would exclude any potential of double recovery. If you tried to bring this matter to small claims to recover you would also be cutting off any right to recover attorney fees. Wage claims are an interesting area of the law where our legislature has actually given private lawyers the ability to obtain their attorney fees from the employer when the circumstances are correct. Therefore it is my own opinion that your first stop should be a visit to an attorney who indicates they do wage claims for a discussion about whether or not there is any reason for that attorney to get involved.