My relative mysteriously died in Bareilly (UP) India. How police investigator can have access to his e-mails?

One of my relatives has mysteriously died in Bareilly, India. Police is investigating it. If they can get an access to his Yahoo-India e-mail box then they may get some clues. My question is how could they request Yahoo company to have an access to his mail box? In other words how they can request for his pword? What's the legal procedure? Please help.