By Mike Dunn

PHILADELPHIA (CBS) — A source tells KYW Newsradio that five years after Comcast opened its new headquarters in center city Philadelphia, the company is actively exploring the construction of one or more new towers nearby to consolidate thousands of workers.

The Comcast Center, at 17th Street and JFK Boulevard, opened in 2008 and already, according to a source, the growing company is out of space and has been for some time.

The source tells KYW Newsradio that Comcast and the Comcast Center’s owner, Liberty Property Trust, are actively exploring the idea of building multiple new office towers nearby.

Liberty Property Trust controls three other parcels between 17th and 19th, JFK Blvd. and Arch Street.

The source said it is premature to give even a time frame for decisions on what might be built. In the source’s words, “There is a long road ahead.”

The source says driving this need for office space primarily is the enormous growth of Comcast’s cable division. Another factor is Comcast’s purchase, completed earlier this year, of NBC Universal, although the source stresses that there is no consideration being given to moving NBC headquarters from New York to Philadelphia.

Comcast’s concept, according to the source, is for a “vertical campus” with several towers within a relatively small footprint of center city.

A spokesperson for Liberty Property Trust, Jeanne Leonard, said only that the company is looking at developing its currently-vacant lot at 18th and Arch Streets, examining its infrastructure capacity, and identifying potential tenants.

In 2008, the previous owners of that site proposed a 1,500-foot tower that would top the Comcast Center, with 63 stories of retail, office space, and a five-star hotel. Amid the recession, that project was shelved, and Liberty Property Trust later purchased the parcel.

Word of the exploration of new sites for Comcast was first reported earlier this week by the blog “Hidden City Philadelphia.” The blog reported that Liberty Property Trust plans to use the noted British architect Norman Foster for a new Comcast tower. Our source says the involvement of Foster is “not off the mark.”