Luigi Mangione, the suspect charged in the fatal shooting of UnitedHealthcare CEO Brian Thompson, wrote in a handwritten note that he acted alone.

Mangione, an Ivy League graduate who was arrested Monday on local charges in Altoona, Pa., authored a three-page document, expressing “some ill will toward corporate America," New York Police Department (NYPD) Chief of Detectives Joe Kenny said. 

“To the Feds, I’ll keep this short, because I do respect what you do for our country. To save you a lengthy investigation, I state plainly that I wasn’t working with anyone,” Mangione wrote, according to law enforcement — who spoke to The Associated Press on the condition of anonymity because they weren't authorized to publicly discuss the investigation.

"I do apologize for any strife or traumas but it had to be done. Frankly, these parasites simply had it coming," he added later, the AP reported.

Mangione was arrested following a tip from a McDonald’s employee in the Pennsylvania town, roughly two hours outside of Pittsburgh. The suspect, who was eating at the fast food restaurant, was found with a gun, silence and fraudulent New Jersey ID card with him.

NYPD Commissioner Jessica Tisch said during a news conference that the fake ID was consistent with the name of the person who checked into a New York hostel. The ghost gun found on Mangione also matched the one used to kill Thompson.

Mangione was charged with five crimes in Pennsylvania following his Monday arrest, according to a criminal complaint. He was accused of carrying a firearm without a license, forgery, tampering with records, false identification to law enforcement and having instruments of crime.

Prosecutors also filed a murder charge against the suspect late Monday, per the AP.