WASHINGTON — Microsoft Corp. attorneys on Friday grilled America Online Inc. chief executive Steve Case about how closely the company had worked with the Justice Department last fall to keep merger negotiations between Netscape Communications Corp. and AOL secret at the height of the software giant’s antitrust trial.

At issue is a conversation between AOL’s then-general counsel George Vradenburg III and David Boies, the government’s lead attorney — evidence, in Microsoft’s eyes at least, that some government witnesses put on the stand at that time may have been less than forthcoming.