He was already being harassed for months prior.

But interrogations can go very badly because the exert enormous mental pressure on people. It's genuinely what they are designed to do. An interrogation weaponizes every available part of self-doubt, insecurity, anxiety, as a weapon.

One effective weapon that interrogations use is silence. You ever get nervous when you were talking about a subject, and the other person just doesn't speak and it freaks you out a bit. Well imagine going through that, and a thousand other pressures, by 2 or 3 people who are trained in making that weapon as painful to you as possible, and the intensity is up by a thousand. These are people who can put enough psychological and emotional pressure on you, that they can actually convince you that you did something you actually know you didn't do. It's not even uncommon, and it's not even special to feds. Your local city police are probably not too shabby at it.

Normally, interrogators desperately avoid letting you kill yourself because it's contradictory to the mission. The problem in the story is that they fucking broke him. He was so mentally broken, he just didn't fucking care, and then killed himself. They put so much pressure on him that he assumed his whole life was over so he decided to end it anyway, even though that wasn't what they wanted.

Unfortunately, that's the real problem here. A false confession from an interrogation can be done so effectively, that interrogators aren't worried about going far enough, but going too far and then invalidating the confession should it ever be reviewed to court.

Interrogators use everything to their advantage, including the size of the room, the temperature and humidity, the aesthetics, literally anything.

I'm not even saying it's all bad. Sometimes, it's good because it can be very hard to convince a murderous sociopath to tell you where the bodies of the children he raped and murdered are. Actually, I distinctly remember watching one interrogation footage of a kidnapping suspect by a police detective and a polygraph assistant (all polygraph assistants are actually just 2nd interrogators, no polygraphs are admissible, it's just a good way to convince you to trust a second interrogator instead of the cop). They managed, after what must have been 2-4 hours to get the guy to confess to the murder, identify where the woman's body was, and then got him to walk them back to the location to find her.

They found her just barely alive. The attempted murderer left her for dead, but she had held on just long enough to be rescued by those very detectives that got her attacker to confess.

It's kind of a Dark Art. Don't trust cops, never trust interrogators, but by god, a good interrogator can do more to get justice for victims and families than any beat cop, prosecutor, or judge ever could.