A prominent Black Lives Matter activist said Tuesday he was stopped by security at Midway International Airport and missed his flight.

The Transportation Safety Administration, however, contends it had no impact on Deray Mckesson missing his departure.

Mckesson, a former mayoral candidate in Baltimore, took to Twitter Tuesday morning after the incident to express his frustration with the Transportation Security Administration.

Mckesson said that a TSA agent stopped him because the agent believed he was moving too quickly. The agent also said he was going to call the police.

“I told him that he can call who he needs to call,” Mckesson tweeted. “He then called his supervisor. I missed the flight.”

In an emailed statement Tuesday afternoon, the TSA said: “After reviewing closed circuit TV video and other available information, TSA determined that our officers followed standard procedures and were not responsible for this passenger missing his flight.”

Mckesson said the supervisor “was as helpful as she could be. But do your agents threaten folks w/ calling the police regularly?”

Sources said Mckesson, whose Southwest Airlines flight was scheduled to take off from Midway at 7:37 a.m., was stopped by the TSA as he was in line at 7:23 a.m. TSA agents spoke with him from 7:26 a.m. to 7:29 a.m.

TSA officers conducted explosives trace-detection swabs of Mckesson’s hands and property at the TSA’s pre-check lane, sources said. Mckesson was not the only passenger who received this screening during the shift.

On its website, Southwest Airlines states: “All passengers must obtain their boarding passes and be in the gate area available for boarding at least 10 minutes prior to your flight’s scheduled departure time.”

Mckesson grew up in the Baltimore area and then worked in Brooklyn through Teach for America. He was a teacher in Minneapolis when a white police officer fatally shot Michael Brown in Ferguson, Mo., in August 2014. After protesting in Ferguson, and documenting the rising anger over race relations, Mckesson decided to return to Baltimore, where he was a candidate for mayor in the primary election.

He ended his Twitter posts on the Midway incident by saying he would file a formal complaint.

“The agent was at the ‘random’ check-point,” Mckesson said in one Tweet. “But he stopped me b/c he had the power to do so. There was nothing random about it.”

Editor’s note: This story was updated to reflect Deray Mckesson is a former Baltimore mayoral candidate. He lost in the primary election.