more breaking news tonight. president trump announcing on twitter that acting homeland security secretary has resigned. he took over after kristen nielsen was forced out in april. on monday he walked off stage during a speech on immigration after protesters had drowned him out with chants of quote immigrants are under attack. last week he also told the "washington post" his concerns for what he called the tone and message for public immigration policy widely seen as an indictment of how the president does those things. i'm joined by phone by national security and justicer correspondent for nbc news. thanks for hopping on the line. >> thanks for having me. what a night, ari. >> what happened here? >> so we understand that mccal exenan resigned.