The e-mails also indicate that this past July, after Mr. Newell told Mr. O’Reilly that a temporary “surge” of additional A.T.F. agents to the Phoenix division was helping them to catch up with a backlog of leads on several investigations, Mr. O’Reilly passed on that information to two other national security staff officials: Dan Restrepo, the senior director for Western Hemisphere affairs, and Greg Gatjanis, the director of counterterrorism and counternarcotics. Mr. Newell had described one of the investigations as a “very large” case involving the Sinaloa cartel, apparently a reference to Operation Fast and Furious.

Fast and Furious, which was led by the Phoenix division of the federal firearms bureau, ran from late 2009 to early 2011. Its strategy of putting suspects under surveillance in an effort to identify a larger network is commonly used in drug trafficking cases. But it ran against the tradition of the A.T.F. for gun trafficking cases, where the priority has instead been on moving as soon as possible to get weapons off the street.

The tactics were controversial among bureau agents, and the case exploded after last December, when a Border Patrol agent, Brian Terry, was killed in a shootout in Arizona. Two weapons found near the scene had been bought by one of the suspects in the Fast and Furious case earlier that year.

Other documents released late Thursday by Mr. Grassley and Mr. Issa shed new light on the reaction by A.T.F. officials in Phoenix and a federal prosecutor working on the case after Mr. Terry was killed. The agents arrested the suspected straw buyer who had originally acquired the weapons within hours of the discovery of the guns near the scene.

However, the documents show, A.T.F. officials and the prosecutor decided to charge the man in connection with his purchase of a different set of weapons. One of the bureau officials wrote that doing so would avoid divulging the existence of “our current case (Fast & Furious) or the Border Patrol shooting case” and “not complicate the F.B.I.’s investigation.”

In a letter to the new acting United States attorney in Phoenix, Ann Scheel, that accompanied the newly disclosed documents about the reaction to Mr. Terry’s death, Mr. Issa and Mr. Grassley demanded internal e-mails from her office and interviews with three prosecutors who work there.

“Since your office directed and approved the daily tactical decisions in Operation Fast and Furious, it is hard to avoid the perception that a conflict of interest exists,” they wrote.