From my experience, Carter has been Shea's second most vocal supporter (Wilson being #1).
I've been involved in meetings and on Boards or committees where issues regarding Shea have arisen. Carter has consistently defended him and gone back to investigate and then reported errors in the accusations or, in those cases where it was warrented, corrections made.
While it is possible his support waned, it would seem to me, rather than blame Carter for Shea's leaving, it would be more accurate to "blame" or give Carter credit for Shea staying around a few extra years.
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It's not where you take things from - it's where you take them to. Jean-Luc Godard