This is, needless to say, a disturbing question; and the only reason I chose to try to offer some answer here is that it is such a serious and disturbing question I thought someone should at least acknowledge you and your question.
I don't think anyone on here can really offer a good enough answer to your question; but I do know that when there is a suicide loved ones can have real trouble believing it was suicide. Sometimes, too, the person who appears to have committed suicide didn't intend to, but, instead, may have accidentally brought on his own death.
Then again, of course, there are times when what looks like a suicide isn't, and when it's really either a homicide or a death caused by accident by someone else.
I would think you should listen to any evidence the police have that indicates suicide, try to keep talking to them, tell them absolutely every possible thing you can think of that may make them further question the cause of her death, and seek out the advice of an attorney who may deal with situations like yours.
Also, finding a trained therapist who deals with people in your situation (people who have lost a child or loved one to what is believed to be suicide) may help you sort out where you should have doubts about whether it was suicide, and what doubts are routinely part of having lost a loved one to it.