Is United Airlines to blame for the terrorist attacks on September 11, 2001? That's the question that a U.S. District Judge is asking.

There aren't too many lawsuits that haven't been settled since 9/11, but some are still pointing fingers as to who was responsible for the death of more than 3,000 people in the attacks on the World Trade Center, the Pentagon and Pennsylvania after terrorists hijacked and used American planes as weapons.

Larry Silverstein, the leaseholder of the World Trade Center property, filed a negligence claim against United Airlines and American Airlines, according to Reuters. He feels that both airlines should be held liable for the loss of property and business.

Silverstein has already received damages from his insurer, but he is looking for more. The hearing of his case was on Thursday and focused on the destruction of 7 World Trade Center which also collapsed after the attacks, according to Reuters.

Silverstein holds United responsible due to suspected security failures which led to American Airlines Flight 11 being hijacked and crashing into one of the towers. Silverstein believes that because of a poor security system, hijackers Mohammed Atta and Abdul Aziz al Omari were easily able to board the plane that was used in the attack.

Atta and al Omari boarded a Colgan Air flight that was headed to Boston from Portland International Airport. The plane landed in Boston, where Atta and al Omari boarded American Airlines Flight 11 which was headed to Los Angeles. The plane never made it to LA. It was used as a weapon to take down the World Trade Center in New York City.

Silverstein says that United should be responsible because the airline operated Portland's only security checkpoint and screened all passengers for all airlines. Atta and al Omari boarded their first flight on the morning of September 11 at Portland.

World Trade Center Properties attorney Richard Williamson said that United Airlines "had to be really vigilant when it did the screening. The first line of defense was Portland.... They were just asleep at the switch," as quoted by Reuters.

U.S. District Judge Alvin Hellerstein, who heard the case, is questioning whether or not Silverstein's claims are valid. In 2009, judge Hellerstein "dismissed claims against other airlines for damages caused by United Flight 175, which also crashed into the World Trade Center," according to Reuters.

United is hoping that Hellerstein follows his previous ruling.

"The duty is to the passengers on our flights," United attorney Jeffrey Ellis said, according to Reuters.

It is not clear when Hellerstein will rule in the matter.

Silverstein wants $8.4 billion in damages, but Hellerstein will allow him $2.8 billion, the cost the Silverstein paid for the leases, if he chooses to not dismiss the case.