FBI agents in Chicago arrested Headley in October 2009 as he was trying to board a flight at O'Hare International Airport. He pleaded guilty in 2010 to all dozen counts against him, including conspiring to murder and maim people in India and plan bombings there. Headley also admitted to plotting the bombing of the newspaper in Denmark.

Headley agreed to testify against Rana in exchange for avoiding the death penalty as well as extradition to face charges abroad.

His testimony is likely to go beyond Rana's involvement and into the highly charged allegation that Pakistani intelligence officials were involved in the Mumbai plot. That could add strain to already delicate relations between the U.S. and Pakistan, experts said.

"Right now is one of the lowest points in the U.S.-Pakistani relationship," Daniel Markey, a senior fellow at the Council on Foreign Relations in Washington, said even before U.S. forces discovered bin Laden last weekend living in a walled compound not far from Pakistan's capital.

Markey, who focuses on U.S. policy in Pakistan and India, called the Rana trial "poorly timed."

"It's another layering on of serious problems that have emerged," he said.

While federal prosecutors link the alleged Mumbai plotters only to Lashkar, Headley has told investigators of a co-conspirator known only as "Major Iqbal," who was working for Pakistan's largest intelligence service, the Directorate for Inter-Services Intelligence, or ISI.

Further, Rana's attorneys have also alleged in court documents that Rana believed Headley was working for ISI.

"I told him about my meetings with Major Iqbal, and told him how I had been asked to perform espionage work for the ISI," Headley said in his testimony to a grand jury, according to a public court filing. "I even told him some of the espionage stories that Major Iqbal had told me."

Major Iqbal, who was added to the indictment last month, was accused of telling Headley to get established in India, open a business and conduct surveillance. At one point, Iqbal specifically asked Headley to target the conference room and ballrooms on the second floor of the Taj Mahal hotel, one of the targets of the attack, authorities alleged.

Pakistan has often drawn sharp criticism and suspicion from U.S. officials that — while accepting billions of dollars from the U.S. — it ignores or supports Lashkar.

Even before the killing of bin Laden, tensions between the U.S. and Pakistan had heightened, experts said, citing U.S. drone attacks in the region and the highly publicized case of a CIA contractor held by the Pakistani government on murder charges after he allegedly shot two Pakistanis.

Now sharp questions are being directed at Pakistan to explain how bin Laden was able to hide there.

While Rana's trial — and specifically Headley's testimony — might be ill-timed for U.S. and Pakistani relations, victims of the Mumbai attacks have been waiting three years to start getting answers about what happened — and who was responsible.

Andreina Varagona, 47, of Nashville, Tenn., who was shot in a leg and arm that day in 2008, said she has forgiven the "children" whom she believes were brainwashed into shooting her and others.

The planners are another matter, though.

"The cowards that were leading these children," she said. "They should be held accountable."