Using Black Mesa assets...?

Haven´t seen this question asked yet, so I´m asking it:

Is the community allowed to use Black Mesa assets (models/materials) when mapping for other source games, like cs :s (obviously giving credit to the Black Mesa developers)?

Thanks - LaBello

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.