The star witness in a terrorism case that could further strain U.S.-Pakistan relations testified Monday that the Pakistani government helped support a terrorist group that carried out a massacre in Mumbai that killed more than 160 people, including six Americans.

David Headley, a 50-year-old drug dealer turned informant who has pleaded guilty to helping plan the 2008 massacre, said Pakistan’s intelligence service ISI worked with the terror group Lashkar-e-Taiba. “They coordinated with each other. The ISI provided assistance to Lashkar,” Mr. Headley testified quietly. Asked what kind of assistance, he said, “Financial and military.”

The testimony came in the trial of Tahawwur Rana, whom Mr. Headley befriended at boarding school in Pakistan. Mr. Rana, of Chicago, has been charged with helping Mr. Headley conduct surveillance ahead of the Mumbai attack as well as an attack on a newspaper in Copenhagen that was never carried out. Mr. Headley agreed to testify against Mr. Rana in exchange for federal prosecutors’ agreeing not to seek the death penalty. Mr. Rana has pleaded not guilty. …

U.S. District Court Judge Harry Leinenweber acknowledged the possible fear of reprisals jurors might feel if they returned a guilty verdict. He ordered the jury empaneled anonymously and forbade court sketch artists from drawing their likenesses.