The officials declined to name the other former military officer in the case, who is suspected as a co-conspirator. He is said to have recently left the Pakistani Army and held the rank of colonel or brigadier general, higher ranks than Mr. Kashmiri held.

Prosecution documents in the case said that the officer was arrested earlier this past summer in Pakistan on unspecified charges and later released. However, another official said that the officer was discharged only after his associates pressured the Pakistani authorities to free him.

In the complaints against Mr. Headley and Mr. Rana, the officer is identified as an uncharged conspirator by the letters “A” and “B.” The complaints describe him as “associated with Kashmiri, as well as with Lashkar-e-Taiba.”

One official who has been briefed on the investigation said that Pakistani authorities had arrested as many as five other people in connection with the plot in recent weeks, including some former or current Pakistani military officials. Those people remain in custody, but it was unclear what role they played in the expanding plot.

American military and intelligence officials said the case involving Mr. Headley and Mr. Rana reflected a new and evolving pattern of individual militants with different backgrounds and experience, rather than terrorist groups, teaming up to plot and carry out attacks.

“In a lot of ways, it’s moving beyond the mainline activities of individual groups to elements of various militant groups or terrorist organizations that have spent time together, have fought together, maybe trained together, that now have associations with certain facilitators that now come together to plan and execute attacks,” said a Defense Department official who is following the case closely.

“The present and future is less about individual groups conducting attacks, and more about combinations of individuals, and groups and facilitators that come together,” said the official, who spoke on condition of anonymity because of the criminal investigation. “Together they have the resources, the means, the insights to execute those attacks.”