CEO killing latest: New York prosecutors presenting evidence to grand jury

Prosecutors at the Manhattan District Attorney's Office have begun presenting evidence to a grand jury as they work to secure an indictment against Luigi Mangione for the killing of UnitedHealthcare CEO Brian Thompson, sources told ABC News.

The DA's office declined to comment due to the secrecy surrounding grand jury matters.

An indictment could strengthen the case for extradition, which Mangione is fighting.

The 26-year-old Ivy League graduate remains in custody at a Pennsylvania state prison after a judge denied bail on Tuesday.

"He has constitutional rights and that's what he's doing" in challenging the interstate transfer, defense attorney Thomas Dickey told reporters on Tuesday.

Mangione was apprehended in Altoona, Pennsylvania, on Monday after nearly one week on the run following the Dec. 4 slaying.

Three shell casings recovered outside the Midtown Manhattan hotel where Thompson was fatally shot match the gun allegedly found on Mangione when he was arrested, police announced Wednesday.

Fingerprints recovered from a water bottle and a Kind bar near the crime scene have also been matched to Mangione, police said.

In Pennsylvania, Mangione faces charges including allegedly possessing an untraceable ghost gun.

In New York, he faces charges including second-degree murder.