Prosecutors interrogated 12 men on Thursday, a day after they were handcuffed and taken off a US airliner that was diverted to Amsterdam because crew and US air marshals believed they were acting suspiciously, a spokesman said.
According to television reports, all 12 men detained were born in Mumbai. However, the Dutch authorities are tight-lipped about their nationality.
"The investigation is still going on and at this moment we do not know anything more than yesterday," said Haarlem prosecutor's spokesman Ed Hartjes.
He declined to say whether the men face terrorism charges.
A US government official said crew members and air marshals on board the Northwest DC-10 bound for Mumbai saw the passengers trying to use mobile phones and passing the phones among themselves while the airliner was taking off. He spoke on condition of anonymity because of the sensitive nature of the subject.
The plane turned back to Amsterdam shortly after passing into German air space and two Dutch F-16 fighter jets were scrambled to escort it on its flight back to Amsterdam.
Prosecutors have three days to bring the suspects before a judge to request permission to hold them for longer. If they are suspected of terrorism, judges can order them held for another 14 days without evidence.
"It was behaviour that average passengers wouldn't do," the official said.
Passenger eyewitnesses gave varying accounts. The 'Algemeen Dagblad' newspaper quoted Nitin Patel of Boston, who sat behind the men in business class, as saying, "I don't know how close we were, but my gut tells me these people wanted to hijack the airplane.
Another passenger, who was not identified, told NOS television he sat next to one of the men and saw nothing suspicious.
A third, who identified herself only as Alpa, said News some of the men appeared to be of South Asian ethnicity.
The alert came two weeks after British police said they foiled a terrorist plan to blow up several US-bound aircraft simultaneously - a plot that sent a chill through the airline industry.
The suspects were held on Wednesday night at a detention centre at the airport.
The captain of flight NW0042 radioed Amsterdam seeking permission to return with a military escort, and jet fighters were scrambled from a northern military airfield.
The national anti-terrorism office said it saw no reason to raise the country's threat level.
In a recording of air control communications, the Northwest pilot declined an offer to put fire engines on standby for the unscheduled landing at Schiphol.
The security alert was the latest among several incidents reported since the alleged terrorism plot was revealed in London. On Friday, a British plane made an emergency landing in southern Italy after a bomb scare, and the US Air Force scrambled jets to escort a United Airlines flight from London to Washington as it was diverted to Boston.
On Tuesday, a flight to New York from Atlanta was diverted to Charlotte, North Carolina, after a flight attendant found a bottle of water and then smelled something suspicious on the plane.