Here's the letter in a nutshell:
After extensive interviews with eyewitnesses, and examination of documents, photographs, videos, and emails, POGO believes that the management of the [local guard force] contract to protect the U.S. Embassy Kabul is grossly deficient, posing a significant threat to the security of the Embassy and its personnel—and thereby to the diplomatic mission in Afghanistan.
The letter documents those allegations pretty persuasively. POGO describes what it calls a "Lord of the Flies" environment that governs the mancamp where the local guard force, two-thirds of them Nepalese Gurkhas, are housed. Lord of the Flies with nearly unlimited guns and ammo is a bad combination.