U.S. Investigation Into Leak of Classified Pentagon Documents Focuses on Insiders

The U.S. government is treating the apparent disclosure of classified material surrounding the war in Ukraine as an insider’s leak, people familiar with the matter say, and is working to identify and apprehend a key suspect in a massive intelligence breach that has exposed the challenges of safeguarding sensitive U.S. information and tested ties with some of America’s closest allies.

Investigators are pursuing a range of leads in the fast-moving inquiry, one of the people said, including some that surfaced as the Washington Post reported late Wednesday that the classified information was posted to a private...