A high-powered committee of Interior Ministry and army investigators missed its Tuesday deadline to submit its report because of the sheer workload from this and other cases, said Abdur Rasheed Khan, who heads the group.
In addition to the investigation of the church bombing, Pakistani and American agents in the past week have been involved in a series of raids against suspected al Qaeda hide-outs, arresting more than 75 people including Abu Zubaydah, one of Osama bin Laden's key lieutenants.
Khan would not say how much longer the committee needs to complete its report on security lapses leading to the March 17 attack in the heavily guarded diplomatic compound in the capital.
A separate team, led by top police investigators, has been assigned to the criminal investigation.
Ehsan Sadiq, superintendent of police, said the team has interviewed scores of people, but has so far made no arrests.
"We are working on important leads and remain optimistic that a breakthrough will come soon," he said, refusing to elaborate.
U.S. Embassy employee Barbara Green and her 17-year-old daughter Kristen Wormsley were killed in the attack along with an Afghan, a Pakistani and a fifth person who has not yet been identified, but is believed to be the lone assailant who hurled grenades into the packed Protestant International Church.
At least 45 people, mostly foreigners, were wounded.
Pakistani police investigators, who do not have access to many modern investigation facilities, have sent DNA samples from the mutilated unidentified body to the United States for tests. A senior police official said authorities are still waiting for the DNA report.
Police suspect that Islamic extremists, outraged by the government's decision to side with the United States in the war against terror, were responsible for the attack. They have said the attack seemed aimed at foreigners in an effort to embarrass President Gen. Pervez Musharraf.
The government has offered an undisclosed reward for any information leading to the identity of the attacker and accomplices, if any.
Pakistan, where hundreds of people are killed every year in religiously and politically motivated violence, has a poor record of arresting those responsible.
Washington ordered the departures of nonessential staff and dependents following the church attack, but said the decision did not reflect a lack of confidence in the Pakistani authorities' ability to protect Americans.
In January, an American journalist, Daniel Pearl was kidnapped and killed in the southern port city of Karachi by Islamic militants. Police have arrested four of the suspects, while seven remain at large.
By Amir Zia