The claim about MS-13 appears to be bogus and would not stand up in court
https://www.lawfaremedia.org/article/abrego-garcia-and-ms-13--what-do-we-know
Abrego Garcia, who has no criminal record in the United States or anywhere else, denies the allegation of gang membership. On March 24, his attorneys filed suit in Greenbelt, Maryland, asking U.S. District Judge Paula Xinis to order him returned. He argued, as the government has since admitted, that his removal was illegal because an immigration judge had granted him withholding of removal to El Salvador due to his well-founded fear of future persecution there from a violent gang known as Barrio 18. His attorneys allege that CECOT houses members of that gang.
This article explores the source of the administrations claim about Abrego Garcias membership in MS-13 and its credibility, both on its face and in the context of the other information known about this individual.....
Abrego Garcias lawyer later tried to obtain more information about the allegations ICE had made at the bail hearing, according to the complaint.
He discovered that the Prince Georges Police Department had no incident report for the arrest, and the Hyattsville City Police Departments report mentioned only the other three men arrestednot Abrego Garcia.
Then the complaint adds yet another disturbing detail:
His attorney also contacted the [Prince Georges Police Department] Inspector General requesting to speak to the detective who authored the GFIS sheet, but was informed that the detective had been suspended. A request to speak to other officers in the Gang Unit was declined.
In the recent Maryland federal court litigation, the government has not contested, through introduction of evidence, any of the specific accusations of Abrego Garcias complaint. It has only, in conclusory fashion, continued to cling to Judge Kesslers finding that Abrego Garcia was an MS-13 member. (Kesslers bond decision was later upheld on administrative appeal in a perfunctory two-page opinion.).....
Shortly after he won withholding of removal status, Abrego Garcia was released to return home. Evidently, no one saw him as presenting any danger to the community anymore.
Since then, once a year, Abrego Garcia has checked in with immigration officials. This is the standard procedure required of individuals with his statusremovable, but with removal withheld from their country of origin. Abrego Garcias last routine check-in occurred on Jan. 2 of this year, without incident.
The trump DOJ does not want to have to defend these claims in a real court hearing