Actor Charles Cioffi died on May 22 aged 90 (Deadline article).

The X-Files fans know him as FBI Section Chief Scott Blevins, who recruited Scully in the 1X79: Pilot. He appeared again in 1X03: Conduit and came back years later in the momentous 4X24: Gethsemane / 5X02-03: Redux trilogy.

I think his character could have been used more in the series, but he was a busy actor in theatre, TV and movies. Most relevant for TXF, he was the main antagonist in one of the big paranoid thrillers of the 1970s, Klute, where he acted opposite Donald Sutherland and Jane Fonda. I am guessing that Carter cast him because of that role.