United Kingdom, Jan. 27 -- Judd Apatow says Rob Reiner's death has been "really tough" on Mel Brooks.

The 58-year-old star has co-directed a new HBO documentary Mel Brooks: The 99 Year Old Man, which features an interview with late filmmaker Reiner, whose father Carl was best friends with Mel.

Speaking about Mel's reaction to Rob's murder, Judd told Extra: "It's family, you know.

"It's a really tough one because as Rob talks in the documentary, you know, he met Mel when he was five years old."

Rob and his wife Michele Singer were found dead at their home last month, and their 32-year-old son Nick is facing two charges of first degree murder over their deaths.

The Reiner family reiterated their faith in the legal system, shortly after...