The key to the federal investigation into allegations Diddy engaged in sex trafficking -- figuring out who was on Diddy's charter jets that went from state to state and around the world.
Federal law enforcement sources tell TMZ ... all of the jet charter companies Diddy used will receive subpoenas. We're told the feds want to look at all the flight manifests to see who was on Diddy's charter jets and where the planes went.
The manifest is critical ... it will enable federal agents to contact the passengers and interview them. They will be particularly interested if some of the people who filed lawsuits against Diddy were on these planes.
The feds will be looking at everyone on these aircrafts -- and not just alleged victims -- because they could be witnesses who either support or dispute the allegations.
Short story ... a slew of people who flew with Diddy will be interviewed ... and that will undoubtedly include numerous celebrities.
This is just one element of the federal investigation. As you know, feds conducted searches Monday and seized a variety of items at Diddy's L.A. and Miami homes. We're told their focus was electronic equipment. We're told the feds have already interviewed several people connected to the lawsuits that have been filed against Diddy.
As for whether the investigation will lead to charges against the rap mogul, that decision, we're told, is going to take time ... because of the scope of the probe.
Diddy has denied all wrongdoing.