After the anti-terrorist operation in Castrop-Rauxel, the Düsseldorf public prosecutor's office applied for arrest warrants against two Iranians aged 32 and 25.
This was announced by the authority on Sunday.
The brothers are accused, among other things, of wanting to obtain toxins for an Islamist-motivated attack.
The security authorities had previously foiled an Islamist-motivated biological weapons attack in North Rhine-Westphalia.
The police did not find any toxins when arresting a 32-year-old Iranian and his brother on Sunday night, as public prosecutor Holger Heming announced on Sunday in Düsseldorf.
However, electronic storage media were confiscated.
The accused is suspected of having prepared a serious act of violence that is dangerous to the state.
The police said he should have procured cyanide and ricin for this.
The prosecutor said there was already a specific tip on Saturday: "The information came from the security authority of a friendly state." As the "Bild" newspaper reports, it is the FBI from the United States.
The attack plans were well advanced.
As with the target of the attack, Senior Public Prosecutor Heming referred to the ongoing investigations and did not provide any information.
We are also in contact with the Attorney General.
This takes over investigations if the plans could have posed a concrete threat to the state.
"The authorities are now investigating at high pressure," said NRW Interior Minister Herbert Reul.
The results would have to be awaited.
Esken: Keep an eye on all extremists
During the night, the police searched the Iranian's apartment with a large number of rescue workers.
The emergency services wore protective suits.
According to media reports, experts from the Robert Koch Institute were also on the spot.
As early as 2018, an Islamist from Cologne wanted to use a ricin bomb.
The Tunisian had already produced a warfare agent with castor seeds before he was arrested with his wife.
Ricin poisoning can be fatal within a short period of time.
The Green domestic politician Konstantin von Notz called for not losing sight of Islamist terror.
"Once again, it is becoming clear that we must not lose sight of or underestimate the dangers posed by Islamist perpetrators, given all the current, very serious threats from militant, well-connected right-wing extremism," he told the Funke media group.
The SPD chairwoman Saskia Esken rejected the accusation that the fight against right-wing and left-wing extremism obscured the view of possible Islamist attacks.
"Islamic extremism is also very much in the focus of the security authorities," said Esken in Berlin.
The Office for the Protection of the Constitution and Interior Minister Nancy Faeser (SPD) would not only have a strong focus on the extremist fringes of the political spectrum, Esken added.