(CNN) Authorities have arrested a Malaysia-based hacker who they accuse of stealing personal information of U.S. military members and giving it to ISIS.

Ardit Ferizi, a Kosovo citizen, was detained in Malaysia on a provisional U.S. arrest warrant alleging he provided material support to ISIS and committed computer hacking and identity theft, the U.S. Justice Department said.

According to a criminal complaint, Ferizi hacked into the computer system of a company in the United States and stole personally identifiable information of more than 1,000 U.S. service members and federal employees. Then, he allegedly gave that information to several ISIS figures, including a prominent propagandist for the group, the complaint says.

U.S. Assistant Attorney General John Carlin called the case against Ferizi -- which combines cybercrime and terror charges as U.S. authorities aim to step up their crackdown on ISIS -- "a first of its kind."

"This arrest demonstrates our resolve to confront and disrupt ISIL's efforts to target Americans, in whatever form and wherever they occur," Carlin said in a statement. ISIL is another name for the Islamic extremist group ISIS.

Ties to prominent ISIS propagandist

The criminal complaint alleges there's probable cause to believe Ferizi hacked into a server and stole names and personally identifiable information of more than 1,300 U.S. military and other government personnel -- a list that was later posted online in August by a group calling itself the "Islamic State Hacking Division."

"We are extracting confidential data," a message from the group said, according to the complaint, "and passing on your personal information to the soldiers of the khilafah, who soon with the permission of Allah will strike at your necks in your own lands!"

The military members' data, including home addresses and photos, was allegedly stolen by Ferizi and passed on to Junaid Hussein, a British hacker who was active on social media recruiting Westerners to join ISIS, authorities said.

The U.S. military announced in August that it had killed Hussein in an airstrike in Syria. He was a leading member of ISIS' so-called CyberCaliphate, which has carried out mostly nuisance hacks on military and other government websites in the United States, France and other countries.

After the list's publication in August, Pentagon officials said they were investigating.

"I take it seriously, because it is clear what they are trying to do," Gen. Raymond Odierno, the U.S. Army chief of ctaff, said at the time.

Many of the phone numbers and email addresses on the list were not in service when tested by CNN in August. But one person on the list, reached by phone, confirmed he had previously served in the U.S. military. He asked not to be named, but said he had recently been notified by the Pentagon that his name and personal information were on the list. Another, reached by email, confirmed she was a government employee who had been warned by the military about being on the list.

Relative: Family is 'shocked' by charges

According to the complaint, Ferizi is believed to be the leader of a hacking group known as Kosova Hacker's Security.

A relative of Ferizi provided CNN with a recent narrative of Ferizi's life and added that the family was "truly surprisingly shocked" to hear of the accusations tying Ferizi to ISIS.

The family is very skeptical of the charges, and the news has been very painful for the family, especially Ferizi's mother, the relative said.

The family member went on to tell CNN that Ferizi's relatives have no ties to ISIS or any criminal organization, and that they "love the USA for what they did in Kosovo, helping my family and my people."

When Ferizi was age 15, he ran into some problems with the police for allegedly hacking a "news portal," the relative said.

But the family wasn't aware "of his title 'hacker,' but we knew that he had good skills in computer science and that's why (a family member) invested in him to cultivate his talents even more," the relative said.

Why Kosovo students go to Malaysia

Ferizi moved to Malaysia to study computer science because of its schools and their expertise. It's not easy for Kosovo students to get into schools in the European Union or the United States, the family member said.

Other students from Ferizi's hometown in Kosovo also attended the same technical school as Ferizi did, the relative added.

Malaysian police said the 20-year-old alleged hacker had entered the country in August 2014 to study computer science and computer forensics at a college in Kuala Lumpur.

Ferizi was arrested while on his way home to Kosovo from Malaysia on September 15, the relative said. The family paid for his ticket to come home because, the relative claimed, Ferizi has mental health issues and he complained to his family that they were worsening so he dropped out of university to return home.

"We were waiting for his return, then suddenly this happened," the family member said.

The family spoke to Ferizi twice while he was in custody with the help of a Malaysian human rights group called Suaram. Both times Ferizi told them he was being accused of being in Syria in 2013, the relative said.

Questions about Syrian travel

The family handed over his old passport to investigators, and the passport contained a stamp for a four-day trip to Turkey in 2013 when Ferizi was 17. He went there with his mother and father to attend a trade fair in Istanbul, according to the relative.

The family was shocked that authorities would pressure Ferizi so much over a four-day trip to Turkey with his parents, the relative said.

The relative spoke with Ferizi on September 23 and then again on October 3 and during the brief conversations, Ferizi said that Malaysian authorities were pressing him hard on whether he traveled to Syria. Ferizi was becoming so tired that he "may accept what they are saying just to come home to Kosovo," the relative said.

Ferizi's family member has been unable to contact him since but said if the court would recognize that he is mentally ill and release him to his family, they could take care of him.

Malaysian authorities had been monitoring Ferizi for a few months after receiving information from the FBI, said Sr. Assistant Commissioner Datuk Ayob Khan Mydin Pitchay, head of the counterterrorism division, Special Branch of the Royal Malaysian Police.

Ferizi was arrested September 15 in Kuala Lumpur, the assistant commissioner said. He is under remand under a provisional arrest warrant while U.S. authorities apply for his extradition.