It says she thought it was a letter to her. This implies that it had her name and address on it.
“Thinking it was a letter to her” does not mean it had her name on it; it might mean only it was in a stack of mail which she received and opened. We have had this issue a number of times on my street where we have groups of mailboxes and the postal service sometimes mixes stuff up (or a sender puts the wrong number on an envelope). We have had people open someone else’s mail while whipping through a stack of their own mail.
Not saying that is definitely what happened here, just that it may be a possibility. Why would her name be on that envelope? If she had any connection with the sender they would know if she was interested in helping with the bomb plot.... it seems unlikely (to me anyway) that she would turn in a friend or family member on this.
But more info would be nice to have to know what went on....