SEATTLE, Washington — The US soldier held for shooting dead 16 Afghan villagers at the weekend was stationed at a military base south of Seattle which has been plagued by scandal and tragedy.

Joint Base Lewis-McChord (JBLM), which could in theory host a court martial for the unidentified serviceman, has only just held a series of military trials over a "kill team" accused of the gruesome deaths of Afghan civilians in 2010.

The sprawling base also saw a spike in suicides last year with 12 people taking their lives, amid questions about how it cares for victims of Post Traumatic Stress Disorder (PTSD).

Home to tens of thousands of servicemen, civilians and their families who live on the base and in surrounding communities, it was dubbed the "most troubled base in the military" by the Stars and Stripes newspaper in 2010.

Jorge Gonzalez, the head of a veteran-run resource center called Coffee Strong near the base, said he was not surprised at the shooting.

"This was not just a rogue soldier: JBLM is a rogue base, with a severe leadership problem. If Fort Lewis was a college campus, it would have been closed down years ago," he alleged in an online statement.

The US soldier behind the latest massacre -- reportedly a 38-year-old father of two -- left the base in December for his first deployment to Afghanistan after three tours in Iraq, the Pentagon says.

Spokesmen for the base have declined to comment on the accused serviceman, but media reports say his family have been moved onto the base in the northwestern state of Washington for their own protection.

He belongs to the 2nd Infantry Division, 3rd Stryker Brigade, according to media reports. Pentagon spokesman George Little said he is "in his 30s, but insisted his name will be withheld until charges are brought.

The Pentagon said the soldier -- whose alleged deeds have sparked warnings of a surge in anti-American violence -- was diagnosed with a traumatic brain injury (TBI) after a non-combat-related vehicle roll-over in Iraq in 2010.

Links have been found between TBI and PTSD, but it is not known if he suffers or suffered from PTSD.

But that has not stopped the re-emergence of questions about the treatment of soldiers for the disorder -- specifically over whether diagnoses of PTSD were altered by military doctors for hundreds of soldiers.

The Washington Post said a probe of the base's medical center is studying assertions by staff members that, from 2007, diagnoses for at least 300 service members were downgraded to lesser conditions.

Some patients claim their diagnoses were changed to avoid the military being responsible for their long-term care, it said, adding that the head of the base's Madigan Army Medical Center is on leave during the investigation.

The base, a key staging post for servicemen heading for Iraq and Afghanistan over the last decade, has only just finished with a series of court martials over a so-called US army "kill team" behind a string of civilian deaths.

The ringleader of the rogue unit, Staff Sergeant Calvin Gibbs, was jailed for life in November over the killings in southern Afghanistan from January-May 2010, in which soldiers cut off fingers as trophies and posed with the corpses.

Three members of the unit had already pleaded guilty in a scandal that threatened embarrassment for the US military on the scale of the Abu Ghraib prison abuse in Iraq, a scandal exposed in 2004.

On New Year's Day this year a 24-year-old Iraq war veteran from Fort Lewis-McChord, who suffered from PTSD, died of exposure after killing a ranger in Mount Rainier National Park in Washington state.