By Ahmed Aboulenein

CAIRO (Reuters) - Egypt said on Friday its navy had found human remains, wreckage and the personal belongings of passengers floating in the Mediterranean, the first confirmation that an EgyptAir jet with 66 people on board had plunged into the sea.

Unconfirmed reports about flight data from the Airbus plane that disappeared while flying from Paris to Cairo in the early hours of Thursday local time pointed to several problems that its veteran pilot may have struggled with minutes before the crash.

"The Egyptian navy was able to retrieve more debris from the plane, some of the passengers' belongings, human remains, and plane seats," the Civil Aviation Ministry said in a statement.

President Abdel Fattah al-Sisi offered condolences for those on board.

The navy was searching an area about 290 km (180 miles) north of Alexandria, just south of where the signal from the plane was lost early on Thursday.

There was no sign of the bulk of the wreckage, or of a location signal from the "black box" flight recorders that are likely to provide the best clues to the cause of the crash.

EgyptAir Chairman Safwat Moslem told state television that the radius of the search zone was 40 miles, giving an area of 5,000 square miles, but said it may be expanded.

A European satellite spotted a 2 km-long oil slick in the Mediterranean, about 40 km southeast of the aircraft's last known position, the European Space Agency said.

Egyptian Prime Minister Sherif Ismail said it was too early to rule out any cause for the crash. The aviation minister said a terrorist attack was more likely than a technical failure, but offered no evidence.

DATA INDICATES SMOKE ALERTS

Although early suspicion centered on Islamist militants who blew up another airliner over Egyptseven months ago, no group had claimed responsibility more than 36 hours after the disappearance of flight MS804, an Airbus A320.

CNN reported on Friday that flight data, from an automatic system called the Aircraft Communications Addressing and Reporting System (ACARS), said smoke alerts were triggered aboard the EgyptAir jet shortly before it crashed.

ACARS routinely downloads flight data to the airline operating the aircraft.

Two U.S. officials told Reuters they could not confirm CNN's report. But they said an electronic sensor system had detected some kind of disturbance outside the jet around the time investigators believe it began falling from cruising altitude.

One of the officials said the disturbance outside the aircraft may have been caused by its sudden and rapid breakup, but it also could have been generated by some kind of mechanical fault or accident or a possible explosion or attack.

The officials asked for anonymity when speaking about the still-evolving investigation.

A screen grab of the flight data transmitted by ACARS to operators on the ground, published on the website of the aviation journal AVHerald.com, indicated failures in the jet's flight control system and alerts related to smoke in a lavatory and the avionics system, minutes before the crash.

The screen grab provided on the website showed only very terse messages sent from the aircraft, such as "SMOKE LAVATORY SMOKE," "AVIONICS SMOKE" and "F/CTRL SEC 3 FAULT."

The U.S. officials said they could not confirm the authenticity the data, however, and EgyptAir officials could not be reached for immediate comment.

Jihadists have been fighting Egypt's government since Sisi toppled an elected Islamist leader in 2013. In October, the Islamic State group claimed responsibility for blowing up a Russian airliner that exploded after taking off from an Egyptian tourist resort. Russian investigators blamed a bomb smuggled on board.

That crash devastated Egypt's tourist industry, one of the main sources of foreign exchange for a country of 80 million people, and another similar attack would crush hopes of it recovering.

The plane vanished just as it was moving from Greek to Egyptian airspace control. Greek Defence Minister Panos Kammenos said it had swerved radically and plunged from 37,000 feet to 15,000 before vanishing from Greek radar screens.

Officials from a number of U.S. agencies told Reuters that a U.S. review of satellite imagery so far had not produced any signs of an explosion. They said the United States had not ruled out any possible causes for the crash, including mechanical failure, terrorism or a deliberate act by the pilot or crew.

Three French investigators and a technical expert from Airbus arrived in Cairo early on Friday, airport sources said.

Adam Schiff, the top Democrat on the U.S. House of Representatives Intelligence Committee, played down comments from U.S. figures including likely presidential nominees Donald Trump and Hillary Clinton that terrorism was the most likely cause.

"At this point, we still can't corroborate the theory that terrorism brought it down or there was some structural problem with the plane," Schiff told CNN.

"Certainly, the backdrop is suggestive of terrorism in the sense that we have the Russian plane in Sharm el-Sheikh and we have the aspiration we've seen time and time again, not only of ISIL (Islamic State) now but of AQAP (al Qaeda), still very potent and still very determined to bring down aircraft.

“But the reality is, we don't have hard evidence that this was terrorism yet."

FAMILY OF PILOTS

Hardline Islamists have targeted airports, airliners and tourist sites in Europe, Egypt, Tunisia and other Middle Eastern countries over the past few years.

Khaled al-Gameel, head of crew at EgyptAir, said the pilot, Mahamed Saeed Ali Shouqair, had 15 years' experience and was in charge of training and mentoring younger pilots.

"He comes from a pilot family; his uncle was a high-ranking pilot at EgyptAir and his cousin is also a pilot," Gameel said. "He was very popular and was known for taking it upon himself to settle disputes any two colleagues were having."

A Facebook page that appeared to be Shouqair's included criticism of the Muslim Brotherhood, repostings of articles supporting President Sisi and pictures of Shouqair wearing aviator sunglasses.

Two former senior crash investigators said the list of possible causes remained wide open and noted there had been cases where deliberate action had been suspected wrongly.

In 1996, a terrorism probe was launched after a TWA jumbo jet crashed off Long Island, New York, on the east coast of the United States, but investigators later found it had probably been brought down by a fuel tank explosion.

The EgyptAir plane that crashed Thursday was carrying 56 passengers, including one child and two infants, and 10 crew. They included 30 Egyptian and 15 French nationals, along with citizens of 10 other countries. The aircraft had made scheduled flights to Tunisia and Eritrea on Wednesday before arriving in Paris from Cairo.

(Additional reporting by Mark Hosenball in Washington; Writing by Tom Brown;