WASHINGTON — Before the FBI announced it had arrested the suspected leaker of dozens of highly sensitive Ukraine war documents — some of which were created to brief the highest levels of the Pentagon — few would have suspected a 21-year-old National Guardsman as the potential culprit.

The FBI nabbed Massachusetts Air National Guardsman Jack Teixeira at his mother’s home Thursday after he was implicated in the biggest national security breach in at least 10 years.

The junior airman is accused of uploading troves of secret and top secret documents to Discord, a social messaging platform for gamers, from which they were spread across several websites.

An airman 1st class, Teixeira held the third-lowest rank the Air Force grants enlisted troops.

His job — a cyber transport systems journeyman — would have had him working not in intelligence analysis, but with IT infrastructure.

So how could such a young, low-ranking individual have gotten access to so many critical secrets?

There are several possible explanations:

Some service members require security clearances as soon as they join the military, Pentagon spokesman Air Force Brig. Gen. Pat Ryder said Thursday.

“Depending on your position, you may require a security clearance,” Ryder said. “If you are working in the intelligence community and you require a security clearance, you’re going to go through the proper vetting,”

Even in his low position — just two ranks above newly enlisted airmen — Teixeira’s job specialty may have required a clearance.

Though officials have not said whether Teixeira had a security clearance, his job as a cyber transport systems journeymen (CTSJ) may have required him to work with and protect sensitive communication channels.

That’s because CTSJs are the Air Force’s “frontline technicians and trainers,” keeping up the infrastructure of the military’s most advanced cyber networks, which are used to securely send and receive highly sensitive information.

“[CTSJs] enhance our capabilities and provide us with the best and most secure systems so we can stay ahead of the curve in everything we do,” the Air Force says in its online description of the job.

Though it’s unclear why a cyber transport systems journeyman would have the need to know highly sensitive intelligence, it’s possible Teixeira’s work with the Air National Guard’s 102nd Intelligence Wing required him to have access to the sensitive channels, if not to the actual intelligence shared within.

Though Ryder declined to say Thursday whether Teixeira’s unit was assigned to support US efforts to help Ukraine, the 102nd provides intelligence for airmen assigned to both domestic and foreign US military efforts.

“Our mission is to provide worldwide precision intelligence and command and control along with trained and experienced Airmen for expeditionary combat support and homeland security,” the 102nd states on its website.

With that mission set, it is possible the 102nd could have been tasked with providing Ukraine-related intelligence.

About two-thirds of US service members are under the age of 30, and most of that group is under 25 — meaning the Pentagon is simply more likely to grant security clearances to personnel barely old enough to legally drink.

As of 2021, nearly 600,000 service members were 25 or younger; 290,000 were between 26 and 30; 200,000 between 31 and 35; and 150,000 were between 36 and 40. Just 100,000 troops — or less than 10% of the military — are age 41 and older.

“We entrust our members with a lot of responsibility at a very early age,” Ryder told reporters Thursday. “Think about a young combat platoon sergeant, and the responsibility and trust that we put into those individuals to lead troops into combat — that’s just one example across the board.”

While the National Guard is best known for its domestic responsibilities, such as responding to riots and natural disasters, the US can activate the military reserve force for foreign assignments, too.

Defense Secretary Lloyd Austin has previously tapped National Guard units to deploy to Europe to support Ukraine in its almost 14-month-old effort.

For example, one of the leaked documents The Post reviewed Thursdaysuggests that a North Dakota Air National Guard reconnaissance squadron has been involved in analysis of Ukrainian drone strikes on targets in Bakhmut, where the most intense recent fighting has been focused.

However, the Pentagon to date has not publicly announced activations of National Guard units to support Ukraine’s war efforts from the US homeland.