Wiktionary has one the definitions of possession as ownership, which I agree that as a simple verb, I posses is tantamount to I own, have. However, the phrase "in one's possession" doesn't effectively or necessarily hold the same meaning for me. It simply implies that something is with you, but not necessarily yours, which is why it often lends itself to accounts of having illicit or stolen property or materials on one's person or in one's home, etc. Wiktionary further cites the following examples under the definition of ownership:
The car is in my possession.
I'm in possession of the car.
This has led to some confusion on a language site I frequent, where one of the users, a Russian, has attempted to use "being in possession of a [type of car - let's say...] Maserati" to translate what amounts to "owning a Maserati" in Russian. Are owning and being in possession of interchangeable?