External reference error message

I've been working on some parts that were made by starting with imported geometry in the form of imported parts. I don't have the original parts
that were imported, and every time I open the file, I get an error message telling me it can't find the files that were imported. When I right click on the imported parts, it doesn't list external references. Can anyone suggest a way to get rid of the error message?
J Kimmel
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