Dutch police released five suspects Friday but kept two others in custody as they investigated an anonymous tip that warned of a plot to plant explosives in a popular Amsterdam shopping area.

Prosecutors said they found insufficient evidence to hold five of the seven people arrested Thursday, and were shifting some of their attention to finding the person who phoned in the tip from Belgium on an unregistered cell phone.

A statement from the prosecution's office appeared to cast doubt on the credibility of the bomb threat.

It said one person still in custody was "being interviewed for offenses other than terrorist activities," and that stolen goods had been found during a search of his home.

But it said the investigation into the terrorist threat was continuing.

The prosecutor contradicted an earlier statement that all suspects were Dutch nationals of Moroccan origin, and said the two still being held have only Moroccan nationality. None of the identities have been released.

The District Attorney was to decide Friday whether and when the two remaining suspects would be arraigned.

The warning came in a call Wednesday night, and appeared linked to the train bombings in Madrid exactly five years earlier.

Police said one of those detained is a relative of an Islamic extremist involved in the Madrid attacks who committed suicide a few weeks later as police closed in.

The Madrid bombings killed 191 people and wounded 2,000.

On Thursday morning, police sealed off the area around a large Ikea furniture store and warehouse, and a nearby street of electronics and sporting goods stores adjacent to the ArenA football stadium.

The stores remained shut Thursday, but were were given the all-clear to reopen Friday. Police kept a strong presence in the area.

A concert by the American rock group The Killers was postponed Thursday night because the venue was near the stadium. The concert was rescheduled for May 29, the band announced on its Web site.

Prime Minister Jan Peter Balkenende said the warning "shows that we must remain alert for threats to our security."

However, the country's threat level remained unchanged at "substantial," the second-highest on a four-step scale, according to the Dutch anti-terror coordination office.

Searches and interrogations Thursday provided no information that a serious threat remained, police said.

Amsterdam Mayor Job Cohen said no explosives were found during the searches.

The terror threat level has been unchanged for months, with experts warning the Netherlands remains a target because of an anti-Islam lawmaker's film criticizing the Quran.

In December, the outgoing anti-terror coordinator Tjibbe Joustra described the threat level as "substantial-plus" because of Geert Wilders' film "Fitna."

The Netherlands has had no terrorist attacks on the scale of the Madrid bombings or London Underground bombings in 2005, which killed 52 people. But intelligence agencies have uncovered several alleged plots by Dutch Islamists, and several are serving jail sentences.

The country has been on alert against Islamic extremism since the 2004 murder of filmmaker Theo van Gogh by an Islamic radical who was enraged by Van Gogh's short film, "Submission," a fictional study of abused Muslim women with scenes of near-naked females with Quranic texts engraved on their flesh.