The Department of Homeland Security (DHS) used an incriminating written statement from the petitioner's ex-wife to prove that he had committed marriage fraud. The DHS did not produce the wife for testimony. The DHS had sought a subpoena for a prior hearing date for the ex-wife, but there was no evidence that it was served on her, and the DHS did not request a subpoena for the final merits hearing. The Court found that the DHS did not engage in sufficient efforts to produce the ex-wife.
The full text of Karroumeh v. Lynch can be found here: http://media.ca7.uscourts.gov/cgi-bin/rssExec.pl?Submit=Display&Path=Y2016/D04-29/C:15-2198:J:Rovner:aut:T:fnOp:N:1745471:S:0