The Pentagon has started culling distribution lists for sensitive national security information after a 21-year-old member of the United States Air National Guard with a junior job was charged in the biggest U.S. intelligence leak in a decade, spokesperson Sabrina Singh said Monday.

Initial findings from the Defense Department’s review of the leak are due in 45 days, Singh told reporters at the Pentagon. It will assess who has access to sensitive information across the world and seek to strike a balance between ensuring that military and civilians have information needed to do their jobs but only what they need to know she said.

The FBI arrested Jack Teixeira on Thursday in connection with the leak of highly classified documents including maps, intelligence updates and assessments of Russia’s war in Ukraine. The U.S. has charged Teixeira with the massive disclosure of intelligence secrets — an embarrassment that has prompted U.S. President Joe Biden to pledge that the administration will clamp down on the spread of classified material, and led investigators to probe whether foreign adversaries played any role in the leak and the dissemination of the material.