Al Mukalla, Yemen ? A terrorist cell sympathetic to the al-Qaida network may have carried out the attack on a French oil tanker in the Arabian Sea, investigators said Friday.

French Defense Minister Michele Alliot-Marie cast off any lingering theories of Sunday’s explosion and fire being an accident, saying “parts of a small boat and traces of TNT were found inside the tanker.”

“France will not be intimidated,” said Catherine Colonna, the spokeswoman for French President Jacques Chirac. “If this does involve an attack, the president of the republic expects Yemeni authorities to do all they can to identify and punish those responsible.”

Specialists from France, Yemen and the United States have been trying to determine what caused the blast and fire that killed one crew member on the tanker Limburg and loosed 90,000 barrels of oil into the Gulf of Aden. The damaged vessel was towed into port here Friday.

Yemen initially tried to dismiss reports the blast was deliberate.

Friday’s disclosure that TNT traces were found on the vessel was the strongest indication yet of an attack similar to that on the USS Cole by an explosive-laden boat in October 2000. That attack killed 17 American sailors and has been blamed on al-Qaida.

A U.S. defense official has said several factors pointed to an attack: the hole in the ship is at sea level, which is consistent with it being struck by a boat, and the Limburg is relatively new, making it unlikely a malfunction caused the blast.

The Limburg’s captain has said a crew member saw a fishing boat approaching shortly before the blast.

Sources close to the investigation said Friday that officials were questioning five suspected Muslim militants. Some 20 fishermen who were on vessels in the tanker’s vicinity Sunday were questioned and released.

Terror experts and Islamic militant sources, meanwhile, said Sunday’s blast was most likely the work of free-lance al-Qaida supporters who may have never been active members of the terror network.

“Al-Qaida is a decentralized organization,” said Alex Standish, editor of Jane’s Intelligence Digest. “It’s like the merchant bank of terrorism, providing financial and logistical support for projects brought to it by small groups or individuals.”