WASHINGTON — A hacker arrested in Malaysia last year pleaded guilty Wednesday to stealing the personal data of US service members and passing it to the Islamic State terrorist group.

Ardit Ferizi, a 21-year-old citizen of Kosovo who was living in Kuala Lumpur, used the Twitter handle Th3Dir3ctorY as part of a collective of hackers from his home country.

Last June, he hacked into a server used by a US online retail company and obtained data of about 100,000 people. Later in the summer, he sent the names, e-mail addresses, passwords, and locations of about 1,351 federal employees and military members to the Islamic State.

Asked by US District Judge Leonie Brinkema on Tuesday why he would ally himself with a violent terrorist organization, Ferizi offered little explanation.

‘‘I don’t know myself why I did this,’’ he said ‘‘I still ask myself why I committed this crime.’’

Ferizi passed the personal military information he uncovered to Junaid Hussain, a British citizen and member of the Islamic State. Last August, Hussain posted links on Twitter to their information with a warning that Islamic State ‘‘soldiers . . . will strike at your necks in your own lands!’’

Ferizi’s case is a milestone, the first brought against a defendant for terrorism and hacking.

Washington Post