The U.S. Army has it's own force of detectives (criminal investigators.) The USACIDC (U.S. Army Criminal Investigation Command, formerly the Criminal Investigation Division)  was first formed during the American Civil War, in order to investigate criminal activity in the army. Despite it's official title, it's still commonly called the CID. Then, as now, it's work includes investigating major crimes like murder, rape, theft, assault and corruption. When the September 11, 2001 attacks took place, there were CID investigators in the Pentagon, and they went right to work collecting evidence. CID investigators went with the troops to Afghanistan, to investigate al Qaeda and Taliban crimes. Later to Iraq, CID investigators were collecting evidence of Saddam's crimes, and looking for terrorists and Iraqis who were still fighting. Much of the success in finding former Saddam commanders, Saddam himself and terrorists in Iraq has to do with the CID personnel and their criminal investigation skills. 

The problem is that there are only 1,500 CID personnel, and they have a lot of work to do. About 750 of the CID people are "special agents." In other words, detectives. There are also a hundred civilian investigators that specialize in procurement fraud. This unit always recovers more money than it costs to operate, and keeps honest a lot of people selling things to the army. There is a smaller unit that specializes in computer crime. 

CID also provides bodyguards for senior members of the department of the army. The Protective Services Unit (PSU) had to add dozens of additional personnel to take care of the increased demand for security in Afghanistan and Iraq. The PSU operates much like the Secret Service does, and works closely with them and similar units in the other services. Most of the senior American officials in Iraq are guarded by the PSU, as are senior Department of Defense personnel back at the Pentagon. 

The CID has been recruiting more new personnel for the past two years, but it's difficult to get people who meet the standards. While the basic qualifications (two years of army service, at least two years of college) seem reasonable, the screening process also looks for those with an aptitude for the work. Moreover, the CID investigators have traditionally been career people. After about a year of training and a few years of field work, investigators are encouraged to apply for Warrant Officer rank, and more specialized training. So it's been difficult to increase CID strength as quickly as the workload has skyrocketed. In peacetime, CID detectives solve over 80 percent of their cases (compared to about 21 percent in civilian police forces.) Part of this is due to the fact the military bases, where most of the crimes take place, are very law abiding places with very helpful populations. But the CID detectives are also very experienced, and very good. With so much of the army overseas, there is less crime at the bases, and troops in combat are too busy or tired to work up much of a crime rate, so it's been possible to ship a lot of CID agents overseas without lowering the quality of work back home. The CID has also called in some investigators from the reserves (many reserve Military Police units contain civilian detectives), as well as civilian detectives. While the CID work closely with  the Military Police corps, it operates as a separate command and reports directly to the Army Chief of Staff.