The 12 Indian nationals were suspected of attempting an act of violence threatening the on-board safety of the plane.
Prosecution spokesman Ed Hartjes said the probe focussed on the passengers' cellphone conversations and fears that their baggage contained explosives.
He said investigation could not conclude that the men intended to commit an act of violence.
The 12 were arrested by airport police at Amsterdam's Schiphol airport on Wednesday.
The Northwest plane bound for Mumbai, India, turned back after the pilot reported concern among the passengers about the suspects’ behaviour.
"Since the September 11 attacks and those that hit Great Britain (in July 2005), the crew and the investigators are on alert. Threats are taken very seriously," the spokesman said. "It was logical to open an investigation on the basis of the captain's indications."
Schiphol airport
The Northwest Airlines Boeing DC10 flight took off from Schiphol with 149 passengers on board, and was over Germany when the pilot reported suspicious behavior.
He returned the plane to Schiphol, escorted by two Dutch F-16 fighter jets. The passengers were then searched, and the 12 detained.
According to one passenger, quoted by the De Telegraaf newspaper, the plane turned around because "the men began telephoning, searching in plastic bags and moving about the plane when it wasn't allowed."