Transportation Security Administration Administrator Peter Neffenger witnessed firsthand the devastation of the deadly bombings at the Brussels airport and subway which killed dozens of people and injured hundreds more.
That tragedy really hit home, Neffenger told members of the Senate Commerce, Science and Transportation Committee on Wednesday.
"I will tell you being there that day, seeing the devastation, seeing the chaos of the airport environment and the evil behind it was a stark reminder of the importance of the work we do every day to protect travelers," Neffenger said.
And that's one of the reasons why his agency is trying to dramatically upgrade its approach to securing U.S. airports.
The fear: someone with easy access to a U.S. airport might try to recreate the Brussels attacks.
Officials say there is no credible intelligence of any plot to conduct a similar attack in the United States, but stress that "we must remain vigilant."
Two million people now pass through the TSA checkpoints at nearly 440 airports every day — figures that greatly complicate the agency's goal of securing the nation's travelers.
So the TSA is redoubling efforts.
The agency has opened a new academy in Georgia which will train 4,800 officers this year alone.
The TSA also has a Law Enforcement Officer Reimbursement Program which provides approximately $45 million each year to law enforcement agencies to beef up their presence and the agency deploys teams of agents and local law enforcement to patrol public areas.
TSA is also keenly aware of the threat posed by airport employees with access to transportation facilities and infrastructure and has stepped up criminal history records checks of aviation workers, among other efforts.
Neffenger has also ordered a detailed vulnerability assessment of the nation's airports. The results of that assessment are due this month.
Congress wants the TSA to go further by adding more dog teams and double the number of armed tactical units, called "VIPR" teams in airports and railway stations nationally. And they want the agency to do more to require airports to limit their access to areas deemed vulnerable and improve perimeter security.
"The only person who's gonna get airports off their duff to limit access into their airports is you," Sen. Bill Nelson, D-Florida, told Neffenger during Wednesday's hearing.
The TSA's increased focus on security comes just two months before the busy summer travel season kicks off. The agency is warning it could be a summer of long lines at TSA checkpoints — and passengers should plan on arriving at their airports early.
In the meantime, the FBI is urging bomb technicians, firefighters, police and chemical retailers to watch for anyone gathering ingredients for a homemade bomb.
"You can get them in a hardware store, you can get them in a supermarket and you can combine them and they'll be quite destructive," Timothy A. Gallagher, a FBI Special Agent in Charge, told NBC News.
As beefed up security provisions continue, the first flights to leave Brussels for U.S. airports landed Wednesday afternoon in Washington and Newark.
In a show of solidarity, Brussels Airlines, Flight 515 was met by Washington Dulles International Airport emergency crews, well-wishers, a water cannon salute on the ramp and Belgian flags for arriving passengers.