Former Valve-owned studio Turtle Rock has reformed and is working on projects again, according to its new website.
The studio was originally founded in 2002 by Michael Booth and went on to develop the first Left 4 Dead game.
It was later acquired by Valve at the start of 2008, however when Valve moved the studio from Orange County to Bellevue, Washington, it led to its effective closure.
According to Gamasutra, Booth has, however, maintained a close relationship with Valve despite declining to transfer to its headquarters, and Valve's Doug Lombardi has said that Booth was continuing to operate a development studio.
The Turtle Rock website describes the company as "developers who worked with Valve Software to help create Counter-Strike: Condition Zero and Left 4 Dead".
It also gives details of a recently released "side project" from the studio - an iPhone application called Garage Buddy.