In this case it's an adjective. One test is to insert "very" before the word. If this is possible, as it is in your example, then it is an adjective.
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In this case it's an adjective. One test is to insert "very" before the word. If this is possible, as it is in your example, then it is an adjective. An example where it is a passive verb would be "The burglars were surprised by the police as they were attempting to open the safe".