In English, when a defining relative clause contains a continuous or passive verb form, we can often leave out who, that or which and the auxiliary. These are called reduced relative clauses and are very common in spoken English.
Example sentences :
Everyone (who is) working in this office suffers from some kind of back pain.
The trilogy (that was) written by Stieg Larsson has been made into a film.
Who is the girl (who is) sitting next to your brother?
Yesterday I read a book (which was) written by Hemingway.
