This may be a duplicate, but if so I didn't see the original.
Recently, unfortunately, this question has become very realistic: If there is someone who knows a close relative is dead but won't be able to bury them for a while; possibly months or even years (for example, the body was kidnapped in to Gaza), does he act like an Onen? Is he Pattur from all Mitzvos, and whatever other rules apply to an Onen?
Note: I can't think of a halachic reason to think it'd be different than a normal Onen. But for some reason my common sense says that he wouldn't be Pattur from all Mitzvos for months or years.