A man detained at a Milwaukee airport after he shook his head at former Sheriff David Clarke on a flight told a federal jury Monday that Clarke's mocking Facebook posts afterward terrified him and he believed he would be harmed for complaining about what happened.

Daniel Black at one point became teary-eyed on the stand, saying he would never file another complaint against an elected official because the incident left him so rattled and that he filed the lawsuit last year because he needs "someone to say this is wrong."

Clarke and Black, 25, were boarding a flight from Dallas to Milwaukee on Jan. 15, 2017 — the day Clarke's beloved Dallas Cowboys were facing the Green Bay Packers in the playoffs. The sheriff was clad in Dallas gear without his trademark cowboy hat and Black said he didn't immediately recognize him because of that. Black said he asked Clarke if he was Milwaukee County's sheriff, according to his lawsuit, and when Clarke said yes, Black shook his head disapprovingly. Black said he made the gesture because Clarke was supporting a rival team.

But Clarke responded by calling deputies to the Milwaukee airport to detain and question Black. He was not arrested or cited, but Black and his attorneys argue that Clarke's actions — particularly his social media taunts — chilled Black's free speech.

"I felt guilty, I felt scared, that I had a target on my back," Black testified, recalling one post in particular on the sheriff's official Facebook site.

Clarke wrote on Facebook: "Cheer up, snowflake ... if Sheriff Clarke were to really harass you, you wouldn't be around to whine about it."

Black said hateful messages on social media followed and he began to think someone would hurt him.

Black's federal lawsuit alleges Clarke retaliated with the posts because Black complained to his office about the plane incident.

Clarke was not at the civil trial, which is expected to conclude Monday. His attorneys said Black did TV interviews after the encounter and didn't appear scared.

"Far from being chilled, he was encouraged and he enjoyed it," attorney Charles Bohl said. He described the case as "an unfriendly internet spat between two people who apparently don't like each other very much."

Black wants a jury to award him a compensation amount that they choose for emotional distress and other damages, as well as attorneys' fees.

Although Clarke is no longer sheriff, the county is paying his legal bills and ultimately will be liable for any damages.

Clarke resigned Aug. 31 to join a political action committee that supports President Donald Trump.