As Greg Lee indicates, participles are commonly considered to remain verbs, despite being used "like adjectives" in many cases.
However, the situation is a bit confusing because, as far as I know, all linguists recognize that participle forms sometimes represent actual adjectives, not verbs. Greg Lee's answer mentioned the "very" test, which indicates that in sentences like "I was very excited about the event" or "He had very swollen feet", the words "excited" and "swollen" are adjectives, despite being identical in form to participles. There are various terms for adjectives of this form; e.g. "participial adjectives" or "departicipial adjectives".
There are a number of tests for distinguishing "participles" (verb forms) and "departicipial adjectives", but different tests don't always agree with each other, and they usually only apply in very specific contexts. As far as I know, there is no way to show that "stolen" is either a verb or an adjective in that sentence. (There seems to be some disagreement in the literature about whether verbal participles can occur in prenominal/attributive position--see my answer to Is “running” a gerund or a participial adjective?--but to me, it seems like they can, so I don't think the position of "stolen" disambiguates its part of speech in "stolen property".)
Looking over the comments beneath my answer there, I just noticed that "carefully" is alleged to only work before participles, not before adjectives. So it seems we can be sure that the word "stolen" would be a participle in a phrase like "the carefully stolen property". But the word "carefully" does not occur in your sentence in its original form, so I'm not sure that the "carefully" test gives us a conclusive answer for your exact sentence.
The existence of the word "unstolen" suggests that "stolen" can sometimes be an adjective, because the negative prefix "un-" typically does not attach to verbs (the homonymous "reversative" prefix "un-" does attach to verbs, but "unstolen" means "not stolen"; it isn't derived from some verb "unsteal"). Based on what I've read, "stolen" would also be considered to be an adjective in contexts like "seem(s) stolen".