The public prosecutor is also examining details of the affair it was confirmed at the weekend, to see whether Niebel should be charged with tax evasion.
The Financial Times Deutschland said over the weekend it would have cost Niebel nearly €4,000 to transport the carpet from Kabul to Berlin had he paid transport firm DHL to do the job.
The minister, who is responsible for promoting responsible government in developing countries, was no longer tolerable, said opposition Social Democrat development expert Sascha Raabe.
He said the carpet was just the tip of the iceberg and that Niebel was, “a minister of cronyism, one who puts his party interests above those of his office in dealing out appointments, and his own interests first when buying private souvenirs,” Raabe told the Rheinische Post newspaper.
Niebel, a member of the Free Democratic Party, has apologized for the incident, telling Monday's Focus magazine, “I have to reproach myself for not having dealt with matters personally.”
He himself declared the affair over, having applied to pay the import tax retroactively. But Chancellor Angela Merkel has not expressed unconditional support for her coalition partner minister. She said on Friday she expected him to clear up the matter as quickly and completely a possible.
His ministry admitted that the 30-kilo, nine-square-metre carpet had been flown from Kabul to Berlin by the BND intelligence service – after Niebel asked its head Gerhard Schindler for a personal favour.
The BND said Schindler had only arranged for the carpet to be flown on his plane to Berlin because he thought it was an official gift.
It also emerged at the weekend that Niebel had chosen the carpet after the dealer was summoned to the German embassy in Kabul to show the minister his wares.
“I wanted to support small business in Afghanistan and buy a carpet for my dining room,” he told Sunday's Bild newspaper. “I actually wanted to go to a bazaar but security did not allow it.”