is a criminal investigation, and there may be, the FAA is promising to cooperate. Now, Senator Chuck Schumer is demanding answers. The question is, why were the controllers forced to continue directing planes while suffering the effects of carbon monoxide poisoning.

Air traffic controllers brought their allegations directly to the Nassau County district attorney this morning, urging prosecutors to open a criminal investigation.

"It just seemed harder and harder to stay awake," said controller Ray Maldonado.

Controllers told us they were so sickened they could barely function, but were forced to remain on their positions for up to four hours directing large commercial jets from the FAA's air traffic control center known as TRACON.

"It slowly started to feel worse and worse," air traffic controller John Conklin said.

"I remember just being extremely fatigued ... very sleepy. It took a lot to stay awake," Maldonado said.

Eyewitness news reported last night that a test of the facility's backup generator sent exhaust fumes into the building's ventilation system, slowly filling the room with potentially-deadly carbon monoxide. Soon, at least half-a-dozen controllers were physically ill.

But the FAA:

Refused to let the controllers leave

Refused to call the fire department to take air readings

Threatened to prevent firefighters from entering the facility, if anyone else called them.

"They were prisoners of that control room!" said TRACON union president Dean Iacopelli.

Union officials say the FAA refused to replace the controllers because TRACON is grossly understaffed -- and closing off the airspace, apparently, was not an option.

"They would have to acknowledge that number one they made a mistake pumping the fumes into the control room. And number two, they didn't have enough staffing to cover ... There would be delays, the airlines would complain. It wouldn't make them look good," Iacopelli said.

Yet that is exactly what the FAA did ten years ago, taking no chances when dust from renovations at TRACON sickened dozens of controllers. There was a partial evacuation then, caused a series of delays and cancelled flights.

Senator Chuck Schumer has been a leading critic of the FAA.

"We've sent a letter to the FAA today asking for a thorough investigation to what's happened. And I have to tell you given my past experience here, the FAA does not have a good record," Schumer said.

In his letter to the FAA, the senator accuses the FAA of trying to cover-up the incident. How else, he asks, can you explain the managers' refusal to call 911? As we reported last night, the FAA is conducting its own, internal investigation.

Now, Nassau County DA Kathleen Rice says her prosecutors are prepared to listen to the controllers.

Air Controller's interviews from Wednesday:

"It was like a fog," said controller Ray Maldonado.

"Sometimes it felt like I couldn't clearly put together something that I normally would have at any other time. You know what I mean?" air controller John Conklin said.

"I remember just being extremely fatigued ... very tired, very sleepy. It took a lot to stay awake that night," Maldonado said.

"Headaches, nausea, dizziness, disorientation, fatigue, inability to focus, not able to concentrate. We had these employees who were in no condition to do this job being directed to continue with this job," said Iacopelli.

"They said, 'no, we're not calling the fire department,'" Conklin said.

"The employees there said, 'Well, we'll call the local fire department and have them come in to test the air. And the operations manager in charge informed them that, 'If you make the call, I will not let them in the gate and I will refuse them entry into the control room,'" Iacopelli said.

Dr. Thomas Carracio runs the Long Island Regional Poison Control Center.

"What's troubling is it sounds like the workers were forced to stay in the area despite the fact they were having symptoms. Hundreds of people die every year with this," Carracio said. "It's something that shouldn't be taken lightly by anyone."

"I can surmise that they weren't relieved because what the FAA felt was, 'Let's keep moving the airplanes and we'll worry about the safety of our employees and passengers later. Maybe nothing will happen.'" Iacopelli said.

"Something potentially dangerous could have happened and thankfully nothing did," Maldonado.