By Timothy Mclaughlin

(Reuters) - A stabbing attack at a Minnesota mall last month that wounded 10 was premeditated and the attacker, who was fatally shot by an off-duty police officer, showed signs of radicalization, authorities said on Thursday.

Dahir Adan, who hailed from a Somali family that had settled in the United States, grew increasingly interested in Islam and withdrew socially before he stabbed 10 people at Crossroads Mall in St. Cloud, Minnesota, on Sept. 17, the FBI said.

The officer who shot and killed Adan will not be charged, authorities said.

"One could reasonably conclude his actions were consistent with the philosophies of violent radical Islamic groups," FBI special agent Rich Thornton said at a press conference in St. Cloud on Thursday where video footage of Adan's attack was released.

In addition, Stearns County Attorney Janelle Kendall said Jason Falconer, the off-duty police officer who fatally shot Adan, was justified in his use of force and will not face charges.

Falconer was shopping at the mall when he was approached by Adan, who asked Falconer if he was a Muslim, according to Kendall. Falconer said that he was not and pursued Adan after he saw the man was carrying two kitchen knives.

Falconer chased Adan into a Macy's store where he shot and killed him after Adan repeatedly lunged at him.

Thornton said the Federal Bureau of Investigation is still investigating Adan's activities and wants access to his cell phone.

"Dahir Adan's iPhone is locked and we are in the process of assessing our legal and technical options to gain access to this device and the data it may contain," Thornton said.

Apple Inc, the maker of the iPhone, earlier this year opposed the FBI's efforts to break into an iPhone recovered from a mass shooter in San Bernardino, California, sparking a fierce debate over privacy and national security.

The FBI was able to unlock the phone with the help of an outside contractor.

Apple did not immediately respond to a request for comment on Thursday.

Following Adan's attack, Amaq, the news agency affiliated with the Islamic State militant group, issued a statement that called Adan a "soldier of the Islamic State," but FBI Director James Comey said last week in Washington at a House Judiciary Committee hearing that this was not definitive proof of a connection between Adan and the group.

"They claimed responsibility," he said. "That isn't dispositive for us because they'll claim responsibility for any savagery they can get their name on."

(Reporting by Timothy Mclaughlin in Chicago; Editing by Matthew Lewis)