he federal judge handling the litigation over the Sept. 11 terrorist attacks opened the courtroom door yesterday for the families of the thousands of people killed or injured in the attacks.

The judge, Alvin K. Hellerstein of United States District Court in Manhattan, ruled that the defendants, who include the airlines and the Port Authority of New York and New Jersey, the owner of the World Trade Center, had a duty to protect the lives of people on the ground, as well as on the hijacked aircraft.

Judge Hellerstein began his long-awaited 49-page opinion simply: "The injured, and the representatives of the thousands who died from the terrorist-related aircraft crashes of Sept. 11, 2001, are entitled to seek compensation."

He continued, "We live in the vicinity of busy airports, and we work in tall office towers, depending on others to protect us from the willful desire of terrorists to do us harm. Some of those on whom we depend are the police, fire and intelligence departments of local, state and national governments. Others are private companies."

Calling the possibility that a hijacked jet might crash and cause deaths on the ground "a foreseeable risk," the judge refused to dismiss the lawsuits already filed on behalf of those killed and injured at the World Trade Center and at the Pentagon — and thereby opened the door to others who have not yet sued.

The defendants announced yesterday that they would immediately appeal the decision, which comes at an early stage in what is sure to be marathon litigation.

"Our hearts go out to all of the families of the heroes of 9/11, including our own Port Authority family members," said Steven Coleman, a spokesman for the authority. "We strongly believe that the responsibility rests with the murderers who led the attacks."

The ruling yesterday has vastly complicated the decision-making process for many families who are facing real but uncertain deadlines for seeking compensation for their losses. New York State has extended until March the deadline by which families must file wrongful-death lawsuits, like those in the court case.

But there is some uncertainty among lawyers and legal scholars over whether families living outside the state can take advantage of that extension. If they cannot, their deadline for filing a lawsuit is today. Several aviation lawyers said yesterday that their offices had been besieged with calls from families concerned about missing that deadline.

"The pleadings are extremely daunting," said Keith S. Franz, with Azrael, Gann & Franz, a law firm in Baltimore that represents a dozen families of Pentagon victims. "It's a task that I don't think anyone will be able to get accomplished in one day. It's a very unfortunate bit of timing."

In response to concern about the court deadline, New Jersey legislators are at work on a bill supported by Gov. James E. McGreevey that would extend that state's statute of limitations to September 2004. It is not clear what steps other states might take to preserve their families' rights to sue if they are not covered by the New York extension.

The other option available to families is to file a claim with the federal Victim Compensation Fund, which offers a quicker and more certain award but which also requires applicants to waive their right to sue. And the deadline for filing with the federal fund is Dec. 22, still very early in the course of the litigation pending before Judge Hellerstein.

The next step in the case will be a motion for summary judgment, explained Prof. Perry Binder at Georgia State University, who has closely followed the cases. And unless the judge accelerates the case, that motion is likely to be heard after the fund's filing deadline, he said.

"The timing was poor, no doubt," he said, but the issues in the case were complex and novel ones.

Both United Airlines and American Airlines said yesterday that they would seek an expedited ruling from the appellate courts that "would provide certainty to the families of the ground victims prior to the expiration later this year of the statutory deadlines" for the federal Victim Compensation Fund. The fund "is the fairest, most efficient method for compensating these individuals," said Todd Burke, a spokesman for American.

Another defendant, Boeing, which made the planes involved in the hijackings, also promised a prompt appeal. "The issue at hand is terrorism, not negligence," said Todd Blecher, a company spokesman.

Larry Silverstein, the leaseholder of the World Trade Center, also said he would appeal. That the towers "stood for as long as they did and allowed many thousands of people to evacuate safely" after the attacks, he said, "is a testament to the strength of the buildings and the brilliance of their design. We neither caused nor permitted these terrorist attacks."

Michel F. Baumeister, a lawyer in New York and a member of the plaintiffs' executive committee in the case, called the ruling "a landmark decision" but cautioned that it still leaves families who decide to sue facing an uncertain future.

In the weeks after the terrorist attacks, Congress enacted legislation that created the Victim Compensation Fund and limited the airlines' litigation liability for all deaths, injuries and property damage to their existing insurance coverage — roughly $1.5 billion for the plane that hit the Pentagon and about $3 billion for the two planes that hit the World Trade Center, he said.

"So even if the families are successful, they may not be able to collect a hundred percent — or any percent — of their damages because there may not be enough money," Mr. Baumeister said.

For that reason, Mr. Baumeister and many other aviation lawyers say that they are counseling most of their clients to seek compensation from the federal fund, which has been granting death awards as high as $6.1 million and averaging about $1.6 million. Generally, only families of very high-income victims or those with large amounts of insurance, which would be deducted from a fund award, are being advised to sue, he said.

At an informational meeting in New Jersey yesterday with the special master of the federal compensation fund, Kenneth R. Feinberg, some families said that they had heard about the ruling but were probably not going to sue. "I don't want to go through litigation for eight years," said Shelli Wright, 33, of Toms River, N.J. Her mother, Sandy Wright, 57, died in the south tower.

Nevertheless, she said, "I think it's going to make people optimistic about filing lawsuits."

At a news conference earlier at the Manhattan law offices of Kreindler & Kreindler, which represents more than 400 families, one widow, Monica Gabrielle, 51, said the ruling had reinforced her desire to pursue her case. "That's the first thing that gives us any hope for accountability and responsibility," said Mrs. Gabrielle, whose husband, Richard, worked at the Aon Corporation.

Mrs. Gabrielle said she was aware that she may lose and end up with nothing. But she added: "For me, it was always about accountability and responsibility. It's not about money. It's about finding answers."