Sometime in 2002 or early 2003
Gene-splicing is the act of taking adaptations from one or more species and applying them to another. If the process is successful the result is Cross-Species Genetics, which will given time lead to a change in physiology, most notably Human-Animal Physiology.
The soldiers were experimented on without their knowledge. They only found out what had been done to them when they suddenly found themselves stronger, faster and more durable, with all their senses enhanced.
The goal was to turn ordinary soldiers into super soldiers and give them superhuman abilities, so the experiment at first was deemed a success. Unfortunately, a flaw was discovered: Any time adrenaline kicked in, the subjects lost control and underwent minor physical changes.
When the problem was revealed, the military tried to kill all the test subjects. Vincent Keller, one of the test subjects, escaped and, realizing that his superiors have ordered his assassination, faked his death and fled. Due to the complexity of the science involved, it is hard to pin-point were it all went horribly wrong, which makes finding a cure very difficult. However, Vincent and J.T have not given up. It has also been implied by various Muirfield agents that Muirfield has been trying to prefect the procedure, presumably to create more super soldiers.