Former Trump national security adviser Michael Flynn has provided “substantial assistance” to the U.S., special counsel prosecutors told a Washington, D.C., federal judge Tuesday in a memo that recommended a nonprison sentence.

Flynn has assisted in “several ongoing investigations,” the prosecutors wrote Tuesday in a heavily-redacted document. They noted his aid in a criminal investigation—although details were hidden behind redactions—as well as special counsel Robert Mueller III's probe into possible coordination between President Donald Trump's campaign and the Kremlin in the 2016 U.S. election. In total, Flynn has participated in 19 interviews with lawyers at the special counsel's office or the Justice Department, the U.S. said.

Prosecutors noted the “usefulness” of Flynn's assistance, writing his “early cooperation was particularly valuable, because he was one of the few people with long-term and firsthand insight regarding events and issues under investigation by” Mueller.