Two people stabbed in a knife attack at the Dutch capital's main railway station on Friday are both American citizens who were visiting the Netherlands, the U.S. ambassador said Saturday as police investigated possible motives, including the possibility it was an extremist act.

Police shot and wounded a 19-year-old Afghan man with a German residence permit immediately after the attack and are questioning him as a suspect.

A dramatic photo captured by a passerby shows two police officers pointing their pistols at a man wearing jeans and sneakers lying on the ground inside a tunnel in the station.

Photo credit: Associated Press

The stabbings occurred shortly after noon (1000 GMT; 6 a.m. EDT) on Friday at Central Station in downtown Amsterdam: "Something happened, we don't know yet what, but during that two people were stabbed and one person with a knife in his hand walked away and he was shot by police," police spokesman Rob van der Veen said Friday.

Central Station is a busy entry and exit point for visitors to Amsterdam, with regular trains linking it to the city's Schiphol Airport. Friday is one of the busiest days of the week for train travel as tourists arrive for the weekend.

Photo credit: De Telegraf

Ambassador Pete Hoekstra issued a written statement saying embassy officials had been in touch with the victims or their families. Police say they have serious but not life-threatening injuries. Authorities did not disclose the identities of the victims.

"We wish them a speedy recovery and are working closely with the City of Amsterdam to provide assistance to them and their families," Hoekstra said.

Police say they are still trying to establish a reason for the attack, including whether it was motivated by extremism.

According to the Associated Press, a statement issued late Friday by Amsterdam's city council said that the Americans did not appear to have been victims of a targeted attack.

Initial police inquiries did not indicate that the victims were chosen deliberately or with a clear reason, Amsterdam authorities said, however "the investigation is still underway, and all scenarios remain open for the investigation team," the City Hall statement said.

Police said in a statement Saturday that the Afghan man was being questioned in the hospital with the help of an interpreter. His identity has not been released, but police say he has a German residency permit.