The Steltek ruled a galaxy-spanning empire long before mankind emerged on Earth. Relics of a Steltek base on Mars were dated to 2.4 billion years of age. Even then, the Steltek were more technologically advanced than humanity had become in the 27th century.
The Steltek eventually retreated from the galaxy, after bringing ruin upon themselves with their technology, possibly in a civil war. Before leaving, the Steltek attempted to destroy all of their technology so that developing races would not bring the same ruin upon themselves. However, some pockets of technology still remain. The Steltek eliminate these pockets when possible.
Grayson Burrows in the Gemini Sector discovered two of these pockets in or about 2669. A Steltek drone, activated by a stray missile fired from Burrows' Tarsus, wreaked havoc on military and civilian forces in the area, destroying the Sixth Confederation Fleet, along with other vessels. After receiving assistance from the Steltek, Burrows managed to destroy the drone using a gun salvaged from a fighter found in the derelict of a Steltek capital ship.
Source: WC CIC Encyclopedia