The raids on Diddy's homes are just the start -- the feds are issuing subpoenas to companies doing biz with him, aiming to identify anyone with info pertinent to the sex trafficking probe ... TMZ has learned.

Sources with direct knowledge tell TMZ subpoenas -- all bearing Diddy's real name, Sean Combs -- were sent this week to various providers tied to the rapper, including his private charter jet, phone providers and computer companies.

We're told the subpoenas were fired off by the U.S. Attorney for the Southern District of New York ... which, as we've reported, is running the investigation.

Our sources say subpoenas will also be issued to commercial airlines, and possibly the FAA, just in case Diddy opted for commercial flights instead of his private jet.

As we reported, the feds' goal here is finding out who flew on Diddy's dime and where they went. They're not only looking for alleged victims, but also any potential witnesses who could support or dispute allegations of human/sex trafficking.

It'll be particularly intriguing to see if people who've sued Diddy were on those planes.

What's more ... we're told the subpoenas will enable feds to gather more intel about the nature of Diddy's involvement with those various businesses.

Goes without saying, this is a crucial aspect of the federal investigation, which began with raids on Diddy's L.A. and Miami homes Monday.

We've been told the feds have already interviewed several people connected to the lawsuits filed against Diddy.

It's going to take time for authorities to determine whether they have enough evidence to indict, due to the expansive nature of the investigation.