It is the FCC[1]. The FAA is only concerned about the launch and de-orbiting as it passes through airspace, and doesn't regulate what happens between, including whether object de-orbit at all. It is odd for the FCC to have this responsibility. They kind of picked it up by default since they were the only agency regulating operations in space in any way, and we had ratified treaties that needed to be enforced, without creating an agency to enforce them.

Edit: The FAA is also proposing orbital debris rules that were supposed to be finalized this year, but aren't released yet[2]. This whole area is a mess, and really needs congress to set clear responsibilities.

[1] https://www.fcc.gov/space/orbital-debris

[2] https://spacenews.com/faa-to-complete-orbital-debris-upper-s...