After anti-migrant protests rocked the eastern city of Chemnitz, Merkel firmly condemned a "hunt against foreigners" backed by videos circulating on social media.
But Maaßen, 55, in an interview with Germany's top-selling daily Bild last week, challenged the authenticity of at least one of the videos, sparking uproar.
For critics, Maaßen's claim played into the hands of the far-right and his attitude was viewed as symptomatic of a domestic intelligence service riddled with far-right sympathizers.
As pressure mounted on him to prove the video was a fake, Maaßen denied questioning its authenticity and said his quarrel was with how the original poster on Twitter had oversold it as a "hunt against people" which he thought was intended to inflame tensions.
He is due to be grilled by two parliamentary committees later Wednesday,
and the leader of the Social Democratic Party Andrea Nahles has suggested he should step down.
The episode has also reopened uncomfortable questions over a service that for long has struggled to escape a lingering whiff of the far-right.
Maaßen in August 2012 took over at the Federal Office for the Protection of the Constitution (BfV) after his predecessor was forced to quit as it emerged the service had shredded files on suspects of the deadly neo-Nazi cell NSU.
As BfV chief, Maaßen leads an agency charged with collecting and evaluating information on efforts to harm the democratic order or which jeopardize Germany's interests.
But among his key tasks following the NSU scandal was also to restore
public confidence in an institution accused of being too lax with the
far-right threat and too heavy-handed on extreme left activism.
Contacts with far right
Married to a linguist from Japan, Maaßen was born in Mönchengladbach close to the Dutch border.
He headed the interior ministry's counter-terrorism team before being appointed domestic spy chief.
Recognizing that wars are increasingly waged in cyberspace, the former
lawyer quickly boosted the BfV's digital armoury.
He has also repeatedly warned against Russian cyber-espionage, including raising eyebrows when he told a parliamentary inquiry that he thought NSA whistleblower Edward Snowden was actually a Russian agent.
But he came under intense pressure following the attack at a Berlin Christmas market in 2016 when Tunisian failed asylum applicant Anis Amri drove a truck into crowds.
According to media reports, Maaßen wrongly claimed his service had no agent in Amri's circles, even though it had a source at a mosque the Tunisian frequented.
But it is his handling of the AfD that has proved most controversial, particularly as he was known to share the far-right party's opposition to Merkel's decision in 2015 to keep Germany's borders open to asylum seekers.
Despite repeated calls for the BfV to formerly place the AfD under surveillance, Maaßen has refused to do so.
A former AfD member has also accused him of having met repeatedly with the party's leaders to advise on how to avoid being placed under surveillance - an allegation Maaßen and the far-right group have denied.
AfD leader Alexander Gauland told journalists this week he had three
conversations with Maaßen about "overall security evaluations". Maaßen did not give him advice, he added.
But Gauland recounted the BfV chief told him "he could turn to him if there were any problems", an offer he said he took up over suspicions of Russian infiltration in the party's parliamentary group.
Heribert Prantl of the Süddeutsche Zeitung noted that "among the worst things that can happen to a top domestic intelligence officer is for him to be accused of sympathy for a far-right party".
"There is more doubt about whether he has put enough distance between
himself and the AfD than whether there has been xenophobic incitement in Chemnitz," Prantl said.
"Given the rather strange news about Hans-Georg Maaßen, one wonders whether the BfV should not take a closer look at its president."
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