Off the top of my head, chapter one of this book covers a ton of this in the northern triangle area of South America specifically - https://www.penguinrandomhouse.com/books/672038/soldiers-and-kings-by-jason-de-leon/

e; Like I said, it’s Byzantine nonsense that nobody with the knowledge to speak to wants to, but a few other sources that start to get at what I’m talking about

• “Northern Triangle: DOD and State Need Improved Policies to Address Equipment Misuse” (arc) is a recent report from a federal agency talking about some of the military aid executive branch agencies are handing out to those countries (and how it seems to be frequently getting misused, but it’s hard to say how frequently because there’s usually no policies on what exactly constitutes misuse or who to report it to)
• This Brookings commentary “Why do US presidents like military assistance?” (arc highlights how hard it is to talk about this stuff, also mentions spending in Colombia and El Salvador in passing

U.S. military assistance often goes by different names, depending on the legal authorities an activity falls under and which department or agency is overseeing or implementing it. These terms include security assistance, security sector assistance, security cooperation, and security force assistance, as well as more niche programs and terms such as security sector governance and defense institutional capacity building.

The Department of Defense commonly uses the term security cooperation while the Department of State uses security assistance. In practice, there is a lot of overlap in roles and responsibilities, with most congressionally allocated funding falling under the Department of State’s legal authorities but executed by the Department of Defense.

A large and unwieldy policy and legal bureaucracy—commonly referred to as the security cooperation or security assistance enterprise—has emerged to oversee, regulate, and execute U.S. military assistance. This entangled web of authorities, permissions, and funding streams makes military assistance incredibly difficult to execute effectively, track transparently, and evaluate.