Area Commander for the Western Isles, Chief Inspector Ian Graham, said: “Our thoughts at this difficult time are with all concerned.”
Hannah, from Gravir, was last seen at about 4pm on Thursday, and when she was later reported missing Hebrides Search and HM Coastguard joined Police Scotland officers in the effort to find her.
There was particular focus on Lews Castle Grounds following a reported sighting of her in the area on Thursday evening, and a body was discovered there yesterday evening.
Hannah had reportedly gone missing after her first day back at her school, the Nicolson Institute.
Hundreds of social media messages have expressed grief and the deepest sympathy for Hannah’s family.
In a Twitter post from Comhairle nan Eilean last night a spokesperson said: “Everyone in the School community will be deeply saddened by tonight’s tragic news.
“School staff, counsellors and other professionals will be on hand on Monday to support pupils. Anyone requiring support over the weekend can contact 01851 705600 OR 07545 886 438”.