A nationwide manhunt is on in Pakistan for a key suspect in the Sept. 11 terrorist attacks, government officials said Thursday.

Pakistani authorities said they're looking for Said Bahaji, 26, who was believed to have returned to Pakistan via Afghanistan. He may have been planning to take a flight from Karachi to Istanbul, Turkey, on Tuesday, but did not show up for the flight, the officials said.

Bahaji, a German citizen of Moroccan origin, is believed to be still hiding in Karachi, the officials said on condition of anonymity. 

Bahaji's arrest would be a major break in efforts to establish a connection between the Sept. 11 attacks and Usama bin Laden's Al Qaeda organization.

He is being named under an international arrest warrant, and German authorities have said Bahaji had close contacts with some of the hijackers who flew commercial airliners into the World Trade Center.

German investigators say Bahaji rented an apartment in Hamburg in 1998 and shared it with Mohammed Atta, a bin Laden lieutenant who was believed to have been the leader of the hijackers.

Pakistani authorities believe Bahaji and three companions traveled to Karachi from Hamburg, Germany, one week before the U.S. attacks. The four — all of whom carried Western European passports but were of Arab origin — spent the night at a Karachi hotel and made calls to Hamburg.

They then traveled from Karachi to the southwestern city of Quetta and were believed to have crossed the border into Afghanistan.

Bahaji confirmed a reservation on the Tuesday flight to Istanbul but did not show up to get the ticket, Pakistani officials said.

In addition, the Pakistani authorities said four people were arrested Tuesday in Karachi while trying to board a Pakistan International Airlines flight to Toronto.

They included two Afghans, one Yemeni and an Egyptian. Officials said the four were believed to have left Afghanistan a few days ago and had been staying in Karachi with a member of the Islamic extremist Sipah-e-Sahaba group.

The Associated Press contributed to this report.