The prime suspect of the shooting at the University of Nevada, Las Vegas, was armed with more than 150 rounds of ammunition, police said Thursday.
According to ABC News, 67-year-old former college professor Anthony Polito is suspected of using a handgun he legally purchased last year to kill three victims at UNLV's Beam Hall.
Police also said a fourth shooting victim is hospitalized in life-threatening condition.
The shooter eventually died in a shootout with police around 10 minutes after shots were first reported.
Clark County Sheriff Kevin McMahill said Thursday that investigators were looking into a motive but noted that Polito applied for several jobs at educational institutions in Nevada but was denied by all of them.
Polito had organized a list of faculty members at schools in Nevada and at East Carolina University in North Carolina, where he served as a professor at the university's business school for 17 years.
"None of the individuals on the target list became a victim," McMahill said.
UNLV President Keith E. Whitfield later identified two of the victims killed as business school professors Patricia Navarro-Velez and Cha Jan Chang. The third victim will be released after their relatives are notified.
Before the shooting, authorities said the suspect visited a post office in Henderson and sent 22 letters to university staff across the country with no return address.
While 22 letters were intercepted, McMahill warned that more could be out there.
"If anybody in the education world receives a letter with no return address that is taped, we ask you to proceed with caution and contact your local authorities," the sheriff said.
Information from The Associated Press was used in this report.