The Army's decision to deny a Medal of Honor for a Green Beret soldier is under investigation by the Pentagon's top watchdog, military documents show.

At issue is why the MOH nomination for Sgt. 1st Class Earl Plumlee, which was approved by his entire chain of command in Afghanistan, was derailed by Army leaders and downgraded to a Silver Star.

Plumlee was nominated for the nation's highest honor for his role in defending Forward Operating Base Ghazni on Aug. 28, 2013, after insurgents breached a perimeter wall with a car bomb and poured into the combat outpost armed with grenade launchers and suicide vests, according to military documents.

Plumlee aggressively maneuvered to kill several insurgents and provided cover while other soldiers safely evacuated the area.

"Despite suffering shrapnel wounds from enemy suicide vests, Staff Sgt. Plumlee continuously exposed himself to enemy fire, rendered aid to wounded comrades and effectively stopped the enemy assault, saving many lives," according to the citation Plumlee was awarded in May.

His nomination for the Medal of Honor was approved by the Afghanistan war's top commander at the time, Marine Corps Gen. Joseph Dunford.

Plumlee "clearly meets the standard ... Staff Sgt. Plumlee's heroism was truly extraordinary," Dunford wrote, according to military documents obtained by Military Times.

Yet after the nomination package was routed through Army officials, it was ultimately downgraded to a Silver Star, an honor for combat valor considered to be two notches below the Medal of Honor.

One unusual factor may have been an Army Criminal Investigation Command probe during the nominations process that targeted Plumlee with an allegation that he tried to sell an illegal rifle scope online, according to an official familiar with the investigation.

But that rifle scope was never government property. The Army investigation resulted in no charges and it is unclear whether Plumlee violated any Army rules, the official said.

The unexplained downgrade prompted criticism from Capitol Hill, in particular from Rep. Duncan Hunter, the California Republican who served as a Marine in Iraq and Afghanistan.

"There are no questions surrounding Plumlee's actions — and it is his actions alone that should dictate whether he is worthy to receive the nation's highest military honor for combat valor," Hunter wrote in a May letter to Defense Secretary Ash Carter.

Hunter suggested the Army's denial of the Medal of Honor may reflect an effort to "politicize the awards process."

On Sept. 2, Deputy Defense Secretary Robert Work sent Hunter a letter saying he has asked the Defense Department inspector general to review Plumlee's medal nomination and its processing to determine "if there were any deviations from the standard procedures for processing such valor awards," according to a copy of Work's letter obtained by Military Times.

The allegations of low-level misconduct regarding the rifle scope may have been a factor in the downgrade, the official familiar with the investigation told Military Times. But other more subtle factors may have affected the decision of Army leaders.

The official said Plumlee is top-notch soldier who sometimes states his opinions bluntly regardless of whether they are politically correct. And that can be a concern for military leaders at a time when veterans honored with an MOH draw both widespread admiration and media attention.

"He respects authority, but he's not afraid of it. There was some concern about how he would 'present' " in public, the official said.

In addition, the Army may have harbored cultural concerns about the decorated soldier, who was once a Marine. Plumlee enlisted in the Marine Corps in 2005 and later moved into Army Special Forces in 2010.

"A lot of people ... were upset by the idea that he was a Marine before he was a Green Beret and that he was not an 'authentic' soldier," the official said.

Army officials declined to comment on the IG investigation and said the decision regarding Plumlee's award followed standard procedure.

"All Medal of Honor recommendations are considered and ratified through an official board process. In the case of Staff Sgt. Plumlee, the Army Senior Decorations Board reviewed all relevant material and unanimously recommended that Plumlee not be awarded the Medal of Honor," said Lt. Col. Jennifer Johnson, an Army spokeswoman.

"The board's recommendation was reviewed by Army senior leadership, who overwhelmingly agreed with the ASDB's recommendation of the Silver Star, which recognizes acts of extraordinary valor," Johnson said.