Andrew and Tristan Tate Under Federal Investigation, Attorney Claims in Docs

7:25 AM PT -- Joseph McBride, lawyer for the Tate brothers, is blasting the investigation ... telling TMZ it's a "pathetic delay tactics by a group of weak lawyers running from a fight in the Florida Defamation Case, because they know the evidence is overwhelmingly on our side."

He adds ... "The SDNY is subject to Washington. Washington is not involved. We are not worried. Nothing changes. Our march to victory continues."

The Tate brothers' legal issues in the U.S. are piling up -- federal prosecutors are now investigating them ... according to an attorney involved in a civil suit with them.

Here's the deal ... Andrew and his brother Tristan filed a $5 million defamation lawsuit against an unidentified woman and 3 other people last year in Palm Beach County, FL -- claiming the defandants' allegations prompted the Romanian officials to criminally charge the Tate bros.

Well, an attorney representing one of the defendants filed legal docs requesting certain information in that defamation case be kept confidential due to an ongoing U.S. Attorney's Office investigation.

In the docs, obtained by TMZ, Danielle Pinter says the document should not be released publicly until the "United States federal investigation and/or prosecution by the Department of Justice for the Southern District of New York of Andrew and Tristan Tate has concluded."

We've reached out to the SDNY for any info on the alleged probe. Ditto for the Tate brothers' rep.

As we reported ... Andrew is dealing with 2 other investigations in the U.S. -- one in Florida and another in Beverly Hills. Tristan is also being looked at in the Florida probe in which Governor DeSantis announced the brothers were not welcome in the state.

TMZ broke the story ... Andrew's ex-girlfriend, Bri Stern, filed a report with police, accusing him of sexually assaulting her last month in the Beverly Hills Hotel. She has also filed a lawsuit against him.

The Tate brothers are also facing human trafficking charges in Romania, and recently returned to that country for a court date in that case. They have not yet been convicted, or found liable, in any of the legal matters they're facing.