(NewsNation) — In the background of one of the largest serial killer investigations the U.S. has seen in recent years is a letter handwritten by the “Happy Face Killer” to the Gilgo Beach murders suspect.

Rex Heuermann is being held in a Suffolk County, New York jail, having entered a not guilty plea to three counts of first-degree murder in the deaths of Melissa Barthelemy, Megan Waterman and Amber Costello. He is also a suspect in the murder of Maureen Brainard-Barnes but has not been charged with her death.

Now, Heuermann has exchanged messages with convicted killer Keith Jesperson, widely known as the “Happy Face Killer.”

Jesperson, who murdered at least eight women across the U.S. in the 1990s, wrote to Heuermann to encourage him to come clean and confess if he’s guilty. He has penned letters to other serial killers and suspected murderers in the past.

Heuermann reportedly replied, in part: “Thank you for your letters and advice. They have been a help and a comfort to me. I do understand what you have said, and I have taken it to heart.” Jesperson sent Heuermann’s response to a podcaster for safe keeping.

There’s no word if the correspondence between Jesperson and Heuermann will continue. Meanwhile, Heuermann’s family is slated to earn $1 million from a documentary about him.

Asa Ellerup, who filed for divorce from Heuermann six days after he was charged, is currently filming a docuseries for Peacock/NBC.

Two independent sources confirmed to NewsNation that Ellerup and her two children, Victoria and Christopher, will receive at least $1 million for participating in the series. As the documentary on Long Island carries on with sightings of production crews, Heuermann and Ellerup’s adult children maintain they have no knowledge of Heuermann’s alleged crimes and are innocent bystanders.

The four women Heuermann is suspected of murdering were sex workers whose remains were among those of 11 victims found near Gilgo Beach in 2010 and 2011. It’s currently not clear whether Heuermann could be connected to some of the other deaths or if they are separate crimes.

Authorities identified Heuermann as a suspect using DNA and took him into custody in July. Investigators seized a “huge list” of items from the architect’s Long Island home.

Heuermann is expected back in court Feb. 4.