Two Dutch fighter jets accompanied the plane back to Schiphol.

"They will be interrogated in the coming hours by police investigators," a police spokesman said.

Asked whether a suspected terrorist plot could be ruled out, he said: "We don't want to get ahead of ourselves".

Dutch police can hold suspects for three days without charge, extended by another three days in special circumstances, news agency ANP says.

The authorities expect to provide more information on Thursday.

Dutch media cited eyewitnesses saying those arrested were not following airline security rules and were exchanging mobile phones that had made other passengers uneasy.

"They were not paying attention to what the flight attendants were saying," an unnamed female passenger said on NOS television.

Security has been increased at airports worldwide in the last two weeks after British police said they had foiled a plot by British Muslims to blow up planes in the mid-Atlantic using liquid explosives disguised as drinks.

But the return of the Northwest plane did not lead to heightened security, an airport spokeswoman says.

She says it also had not affected other flights at Schiphol, Europe's third largest cargo airport and fourth biggest passenger hub.

Dutch airport officials says the Northwest pilot decided to turn back his 273-seat DC10-30 when it was in German airspace.

A Northwest spokeswoman says the passengers who were allowed to go free were staying in local hotels, adding Northwest would try the flight to Mumbai again on Thursday.

- Reuters

Suspects Indian, to be released without charges

ALL 12 PASSENGERS detained by the Dutch police, following a "security alert" aboard a Mumbai-bound airliner, are Indians. But a day after the dramatic return of Flight No. 42 of the US's Northwest Airlines to Amsterdam's Schiphol airport, the reason for detention is not terror-related.