CHICAGO — More details about plotting behind the deadly 2008 Mumbai attacks — and alleged involvement by a Pakistani militant group and the country’s largest intelligence agency — were expected to emerge Tuesday as the government’s star witness returned to the stand in the federal terrorism trial of a Chicago businessman.

The trial of businessman Tahawwur Rana is being closely watched around the world for what the attack’s scout — Rana’s longtime friend David Coleman Headley — might reveal about possible links between the anti-India militant group, Lashkar-e-Taiba, and Pakistan’s Inter-Services Intelligence agency, known as the ISI.

Headley already has pleaded guilty to laying the groundwork for the Mumbai attacks, and he agreed to testify against Rana to avoid the death penalty, making him one of the most valuable U.S. government counterterrorism witnesses. His testimony was expected to resume Tuesday.

What Headley says during the trial could inflame tensions between Pakistan and India and place more pressure on the already frayed U.S. and Pakistani relations. His testimony also could add to the questions about Pakistan’s commitment to catch terrorists and the ISI’s connections to Pakistan-based terror groups, especially after Osama bin Laden was killed by U.S. forces in a military garrison town outside Islamabad earlier this month.

The Pakistani government has denied the ISI orchestrated three-day siege in Mumbai that left more than 160 people dead, including six Americans. Pakistani intelligence officials have not commented on the trial.

Headley, a Pakistani-American, detailed for jurors Monday how he spent months making videotape surveillance and recording GPS locations of sites in Mumbai, including the Taj hotel and boat landings. He testified that he also traveled to Pakistan numerous times to debrief with insiders of Lashkar-e-Taiba and ISI.

“They coordinated with each other and ISI provided assistance to Lashkar,” said Headley, who told jurors he first started training with Lashkar in 2000.

Headley said that in Pakistan he met with a man by the name of “Major Iqbal,” who was described as working for the ISI, and with Headley’s Lashkar handler, Sajid Mir. Both Pakistani men, along with four others, are charged in absentia in a federal indictment, but Rana is the only one on trial. He has pleaded not guilty.

Prosecutors allege that Rana, a Canadian citizen who runs a Chicago-based immigration and law services business, let Headley open an office in Mumbai and travel as a representative of the company while he scouted for the attacks where gunmen arrived by boat for a three-day siege. Rana is also accused of helping arrange travel and other help for Headley, who planned an attack that never happened on a Danish newspaper. The Jyllands-Posten newspaper in 2005 printed cartoons of Prophet Muhammad that angered Muslim worldwide because pictures of the prophet are prohibited in Islam.

Rana and Headley, both 50, met as classmates at a prestigious military boarding school in Pakistan and have stayed in touch. Defense attorneys told jurors their client was taken advantage of by his longtime friend and did not know what was in store. But Assistant U.S. Attorney Sarah Streicker said Rana was not duped and knew of the plans, both in Mumbai and Denmark.

Defense attorneys were expected scrutinize Headley’s credibility as a witness, saying he has been motivated to change his story and that he was working for the U.S. government even as he said he was working for Lashkar and ISI.

Headley, born Daood Gilani in the U.S., has also been an informant for the U.S. Drug Enforcement Administration after a drug conviction.