The fact that Homeland Security is involved is a huge clue in what direction law enforcement is looking. The question is what services do they provide that the FBI doesn't? That being said, DHS may have been used to eliminate the cartel theory.
To solve this case, you need to shy away from the bizarre (bed bugs and 🤬🤬🤬🤬 fighting cartels that don't exist), and refocus on the basics.
1. Who would want them dead? (money?)
2. Who would gain from having them dead?
3. Who had a grudge.
No such this as a 🤬🤬🤬🤬 fighting cartel.