I think Jeff should be singled out as a suspect. Although, he does seem very considerate with his statements, he still admitted being there in person at the same office, during the same evening where Annette was murdered. Conjointly, he was the only one. It's possible that as he was about to leave the parking lot that night, he spotted Annette pulling into the parking lot, and could have waited in his car for her to go into the office, snuck back in, and shot her with the nail gun. With this scenario, you're probably asking yourself, why? What would drive Jeff into doing such a terrible thing?
Maybe his wife, Elena, set a motive for him to kill Annette because she was finally fed up with being jealous of her/with Annette still playing a major role in his life. Maybe she threatened Jeff, saying that if he wouldn't do anything about it, she would.
I also think that Elena and Joanna, her best friend, should be singled out as suspects. As I recall from Meg's interview and partially Kurt's interview, it was clearly stated that Elena was jealous of Annette because of a past fling she had with Jeff. Meg also stated that she overheard Elena tell Joanna that the interaction between Annette and Jeff had to be stopped. Joanna then coincided with Elena without hesitation. There is another possibility that Elena could have murdered Annette herself, or could have sent Joanna to kill Annette for her.
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