WASHINGTON — The United States is poring over a vast trove of new intelligence about Islamic State fighters who have flowed into Syria and Iraq and some who then returned to their home countries, information that American officials say could help fight militants on the battlefield and prevent potential plotters from slipping into Europe.

American-backed Syrian Kurdish and Arab militias have seized more than 10,000 documents and 4.5 terabytes of digital data in recent weeks while fighting insurgents in Manbij in northern Syria, near the Turkish border, a major hub for Islamic State fighters entering and leaving Syria, American officials said.

An initial American review of the material offers new clues about “foreign fighters, the networks, where they’re from,” according to Brett McGurk, President Obama’s special envoy for combating the Islamic State. Other officials said the information included the fighters’ identities, countries of origin, routes into Syria and the illicit networks that recruited and ferried them to the region. Those details are being shared with allies to help stanch the flow of militants.

“We want to make sure that all that information is disseminated in a coherent way among our coalition partners,” Mr. McGurk said last week, during a meeting of foreign and defense ministers in Washington, “so that we can track the networks from the core and all the way to wherever the dots might connect, whether that is in Europe or in North Africa or Southeast Asia.”