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SALVADOR, Brazil – FIFA officials likely will conduct an investigation to explain why hundreds of seats were mysteriously empty at the Netherlands' 5-1 victory over Spain, according to a source at soccer's governing body.

Clusters of unused seats were visible at Arena Fonte Nova on Friday despite the game being one of the World Cup's most anticipated of the group stage and featuring the two finalists from the 2010 tournament. According to FIFA, the game had been completely sold out months in advance.

"It is definitely something that will be looked into," the source told Yahoo Sports. "It is unusual when something out of the ordinary like this takes place."

Horrific traffic could be blamed for the no-shows. It began as early as 11 a.m., a full five hours before the match, as taxis to the stadium were not permitted to get any closer than FIFA's perimeter zone – a radius of approximately two kilometers (around 1.25 miles) that only allows official vehicles to pass through.

A local vendor said things only got worse later on.

"Nothing was moving," Marcela Pinta said with the help of an online translation service. "Walking was quicker."

Given the doubts raised about the capacity and effectiveness of the regional airports in Brazil, most flights into the Salvador airport on Friday were surprisingly running on time or close to it.

FIFA also figures to look into whether a ticketing mix-up took place. Some fans were surprised to see tickets in some areas of the stadium to become available online again as recently as last week. However, when prospective buyers tried to purchase them, their attempts were rejected by the official World Cup system.

Another explanation for the empty seats could be that an official World Cup sponsor accepted its full allocation of complimentary tickets but chose not to use them. But it is unlikely that sponsors could be responsible for all of the empty seats, given the way that the blank areas were scattered around the stadium.

The Netherlands would have loved to use the available seats to pack as many extra fans as possible. Spanish supporters might not have been grateful for that fact.

"Our fans are amazing to us," said Dutch substitute Dirk Kuyt, who admitted he had been too focused on the game to notice the empty seats. "They are like a secret weapon. It doesn't matter where it is, South Africa or Brazil. They are everywhere and they are orange."