American ISIS hostage Kayla Mueller sent this heartbreaking letter before she died

The family of an American hostage who was recently killed in Syria has released a heartbreaking final letter she wrote before her death.

US government officials confirmed the death of Kayla Jean Mueller, an aid worker held by Islamic State militants in Syria, on Tuesday. She was 26 years old.

In her letter, she said she asked her former cell mates to contact her family and send the letter. Mueller said she was in a safe location and healthy.

"Just the thought of you all sends me into a fit of tears," she wrote. "If you can say that I have 'suffered' at all throughout this whole experience it is only in knowing how much suffering I have put you all through; I will never ask you to forgive me as I do not deserve forgiveness."

Mueller had been held in Syria since 2013. The Islamic State terror group, also known as ISIS, claimed that a Jordanian air strike killed her last week, but the group released contradictory evidence to back up the claim.

Here's a photo of the letter (BuzzFeed has a transcript):

Mueller family release final letter. "If you are receiving this I am still detained but my cell mates released" #ISIS pic.twitter.com/OpJDWmK2TX

- Jon Williams (@WilliamsJon) February 10, 2015

Government officials reportedly told CNN that it's unclear how Mueller died. She was the last American known to be held by ISIS.