He appears in videos shrouded in black or with his back facing the camera. He doles out advice on his Facebook page to hundreds, if not thousands, of followers. And he is suspected of being one of Germany's top recruiters for the so-called "Islamic State" (IS) terrorist group.

Ahmad Abdulaziz Abdullah A., better known as Abu Walaa, is thought to have been in contact with the man who allegedly plowed a truck into a Berlin Christmas market on Monday night, killing 12 people and injuring 49.

Authorities are still unclear as to how, when or where Anis Amri, the Tunisian refugee suspected of carrying out the attack in Berlin, might have been radicalized. But they believe the 24-year-old, who was killed in a shootout with Italian police on Friday, had made contact with members of Germany's ultra-conservative Salafist scene, possibly including Walaa.

Federal Criminal Police chief Holger Münch, the official who earlier this week floated Walaa's name in connection with Amri, didn't specify how exactly the two men might be linked. But his assertion has raised questions about the influence of so-called "hate preachers" like Walaa, within Germany.

'Preacher without a face'

The 32-year-old preacher moved to Germany from Iraq in 2000, originally settling down with his family in the town of Tönisvorst, in North-Rhine Westphalia. Over the years he established himself as one of the most influential Salafists in the country, preaching from a mosque founded in 2012 in the northwestern state of Lower Saxony.

Called the "preacher without a face" due to videos posted online in which his face was always hidden from view, Walaa built a strong social media following that at one point amounted to as many as 25,000 Facebook fans. In his videos, the preacher could be heard sharing his words of wisdom on such mundane topics as how to maintain a healthy marriage.

"Most marriages would be solved if only the spouses didn't argue about unnecessary things," Walaa advised in one video, according to an article on "Spiegel" Online. "Often times a trifling matter escalates into a big dispute."

Homegrown terror?

But even as Walaa was engaging in trivial matters like marriage counseling, German authorities were keeping a close watch on him. His mosque had become one of the centers of the country's Salafist scene, and around 20 men who left Germany to join IS in Iraq and Syria were thought to have been radicalized there. In the summer of 2016, police conducted a search of the mosque, though no arrests were made.

Later, on November 8, Walaa was arrested along with four others on suspicion of establishing a network to recruit German IS fighters. The preacher "pledged allegiance to the so-called "Islamic State" and appeared as a speaker at numerous Salafist events in the past," said the federal prosecutor's office in Karlsruhe following the arrest. "The goal of his respective network was to recruit people to join IS in Syria."

However, as German broadcaster n-tv reports, a current lack of evidence against Walaa is a hindrance when it comes to proving some of the more serious charges against him, such as allegations that his mosque has donated money to IS.

'May Allah protect us'

Walaa's arrest was also praised by some within the Salafist community. The preacher had engaged in an online spat with one of Germany's most recognizable Salafist figures, Pierre Vogel, who has openly condemned IS and warned his followers against engaging in terrorist activities.

The day Walaa was arrested, Vogel posted a message to his followers on Facebook: "May Allah protect us from the evil of Abu Walaa and his lies."

According to the government, nearly 800 people left Germany in 2015 to travel to Syria and Iraq to join terrorist groups like IS. How many of these cases can be linked to "hate preachers" like Walaa is unclear. And while it has yet to be determined if there's a firm connection between Amri, the suspected Berlin attacker, and Walaa, the suggestion points to a growing concern over homegrown terrorism in Germany.