The killers brought the bags and tape with them. A bag with tape was found next to the bodies. Really I think they would be dead in less than 10 minutes with the bags tight around their necks. (probably about 6 minutes) But they had injuries to the neck area, so strangulation is a possibility too. I really don't see a way that someone could tie a plastic bag around someone's head and think that they could live. Sounds like premediated murder.