I'd use "were", too because percentage is related to escape attempts.
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GPY It may be that in the US "a small percentage" is seen as grammaticallly singular, so requiring a singular verbThis American prefers to see it as grammatically singular, but I fear I am in the minority on this one.
AnonymousIt is not known how many escape attempts were made, but only a small percentage (of the attempts) were successful.I would leave it as above. My reasoning is shown in parentheses.