Kang was first deployed in 2010 to Iraq.
By late afternoon Monday the elder Kang was still struggling with the news.
After taking the IS pledge, Kang told a confidential source that he wanted kill "a bunch of people".
The FBI suspected Kang acted alone and investigated him for over a year before arresting him.
Accordng to KGMB-TV, Kang was the subject of a year-long investigation by the FBI.
Kang, 34, was employed as an Army soldier at Schofield Barracks in Hawaii, Laanui said. He, then, pledged his loyalty to ISIS.
Kang also is accused of attempting to furnish ISIS with classified and other sensitive military records that would have assisted the group in its combat tactics.
He even relayed to the FBI informant in March that he researched the most painful and effective ways "people had been tortured".
Kang has two firearms registered in his name, an AR-15-style assault rifle and a handgun.
He had been under investigation by the US Army and the FBI for over a year. The soldier was briefly into mixed martial arts, competing in a couple of tournaments in Kansas City, Mo., while stationed at Fort Riley. Now red evidence tape could be seen on Ikaika's condo in Hawaii, and Kang's photos are spreading online.
Kang enlisted in the Army in December 2001, just months after the Sept. 11 attacks.
Clifford thinks his son might be suffering from post-traumatic stress disorder (PTSD) from his tours in Iraq and Afghanistan.
Pua Edayan, the office manager at Kang's condo complex in Waipahu, described Kang as " a quiet person". He entered no plea.