It is not essential to have witnesses in order to enter Islam

My question is one concerning, when a non muslim wants to accept the Shahadah.  Do witnesses need to be present?  I remember a hadith  mentioning a man who killed 99 ppl and was sent to heaven b/c of his intentions of coming towards islam?

Praise be to Allaah. 

It is not essential for the one who wants to utter the twin testimony of faith (al-shahaadatayn) to have witnesses present. He may simply utter the words with his tongue, and that is sufficient for him to enter Islam.