I found the following information at https://www.thoughtco.com/reduced-relative-clauses-1211107
I gave a book which was written by Hemingway to Mary last week.
The second example sentence cannot be reduced because the relative clause "which was written by Hemingway" modifies an object of the verb 'give'.
However, I found the following information with some examples at the following site http://esl.fis.edu/grammar/rules/relative.htm
When the relative clause contains a present or past participle and the auxiliary verb "to be", in such cases both the relative pronoun and auxiliary can be left out:
Here, the following example appears to contradict with what is mentioned at the former site:
She was wearing a dress (which was) covered in blue flowers.
I want to know which one out of the above two is correct.