New details have surfaced in the case of the Princeton man accused of killing his younger brother on Feb. 22.

Matthew Hertgen is charged with first-degree murder in addition to several other charges in the killing of his younger brother, Joe Hertgen.

The affidavit from the Mercer County Prosecutor's Office shares gruesome details from the night and information leading up to the 911 call.

In the document, the investigation revealed information shared by a third Hertgen brother. He says he was with Matthew earlier on the evening of Feb. 22. He shared Matthew had suffered from severe mental illness for the past five years and on Saturday, was extremely distressed, despondent, and experiencing terrifying visions. He spent the evening with him, wanting to help.

Later Saturday night, the third brother texted the victim, Joe Hertgen. In the message he shared Matthew was struggling with his visions and contacted him immediately if he needed help. Joe responded, and sent a text back saying "will do," and invited the brother to a hike the following day.

That text was sent around 10:30 p.m. About 45 minutes later is when Matthew called 911 and reported his brother, Joe Hertgen dead.

Matthew Hertgen told police he "went into a fit of madness".

Matthew Hertgen first appeared before a judge last week.

Now, he's expected to appear in person at Mercer County Court later today.