Co-leader Alexander Gauland said the BfV agency had refused to give him a copy of a report justifying its recent decision to monitor the party more closely.
According to German weekly Der Spiegel, the confidential report concluded that Gauland used some of the same aggressive language as a party faction known for its ethno-nationalist rhetoric. Security officials say the faction, 'The Wing,' and the party's youth branch will be scrutinized particularly closely in future, including through the possible use of covert surveillance.
"I've been warning about possible surveillance by the BfV since the start of 2016," said former leader Frauke Petry, who left the party in 2017, citing what she described as "totalitarian structures" emerging from the Wing.
Gauland and others had "spectacularly underestimated and downplayed" the risk, Petry told The Associated Press.
"It's sad to see what's become of AfD," she added.
Meanwhile, the party refused to comment Friday on reports that it has handed parliament a list of 14 individuals it claimed were behind a 130,000-euro ($146,500) donation it received from Switzerland before the last election.
German law only allows parties to receive funds from European Union citizens.
German and Swiss media reported that the donors were all registered in the EU, raising the question of why the money was funneled through a non-EU country.
"AfD's statement of accounts has as many holes as a Swiss cheese," said Ulrich Mueller, an analyst at transparency group Lobby Control. He called for an overhaul of Germany's system of party financing to prevent donors from secretly influencing parties and elections.