Police say 18-year-old Sulejman Talovic was "heavily armed" when he killed five people and injured four others at Trolley Square on Monday evening.
Salt Lake Police Chief Chris Burbank said Talovic carried a .38-caliber pistol, a shotgun and a backpack filled with ammunition, plus a bandolier around his waist fixed with shotgun shells.
Burbank said Talovic's tactics showed he was out to kill as many people as he could.
In the aftermath, authorities are investigating how Talovic, described by neighbors as a quiet young man who kept to himself, obtained the firearms he used to cause such tragedy.
Police have reported Talovic and his parents came from Bosnia. As of Tuesday, officials with U.S. Immigration and Customs Enforcement reported Talovic was a legal resident alien who had been issued a "green card" when he and his family immigrated to the United States in 1998.
"He is a legal permanent resident of the United States," said ICE spokeswoman Virginia Kice. Although his nationality is listed on immigration documents as Croatian, one of Talovic's relatives said they are Bosnian.
Under Utah law, illegal aliens are forbidden from purchasing firearms; however, legal resident aliens can legally purchase firearms.
"A person who has legal status in the country can possess a firearm under federal law," said U.S. Attorney for Utah spokeswoman Melodie Rydalch. Generally a lawfully permanent resident can purchase firearms, but there are some exceptions imposed by the government on a case-by-case basis, Rydalch said.
Currently, federal officials cannot say if Talovic, being 18, was restricted in any way.
Federal officials are also assisting Salt Lake City police in tracing the origins of the shotgun and handgun.
"We've offered our assistance to Salt Lake police in any way, shape or form," said ATF Resident Agent in Charge Lori Dyer. Currently ATF is using the make, model and serial number of both weapons to trace their origin back to the manufacturer. From there, Dyer said, they can trace the gun to the original retailer and from there who the firearms were first purchased by.
Dyer said that will not necessarily lead law enforcement to Talovic, who could have purchased or taken the firearms from someone else. Dyer said a bit of detective work will have to be done in order to continue the firearm trace.