A court ordered the teenager, identified only as Paweł G. under Polish privacy laws, to be held in pre-trial detention for three months.
Żurek said weapons, explosives and Nazi symbols were found at the teenager's home.
According to the National Public Prosecutor's Office, prosecutors have charged Paweł G. with three offences allegedly committed between May 28 and September 20.
The most serious charge concerns "preparing a terrorist attack intended to kill multiple people," it said.
The other charges involve alleged participation in an organised criminal group whose aims included committing terrorist offences, promoting Nazism and fascism, and inciting hatred.
Prosecutors said evidence gathered during the investigation, particularly searches of the teenager's home and examinations of his phones and other electronic devices, indicated a high probability that he had committed the alleged offences.
Investigators found knives, a bayonet for an AKM-type rifle, books, patches and other materials bearing Nazi and fascist symbols, the National Public Prosecutor's Office said.
An analysis of data from the seized devices also uncovered communications concerning the alleged planning of mass attacks, including attacks on a school and a mosque, surveillance of potential targets and methods of carrying out attacks, as well as efforts to obtain weapons, ammunition and military equipment, prosecutors said.
The communications also discussed the use of grenades and Molotov cocktails, they said.
Paweł G. denied the allegations and refused to give an explanation, according to prosecutors. A court subsequently ordered his three-month pre-trial detention at the prosecutor's request.
He faces up to 15 years in prison if convicted of preparing to kill multiple people, prosecutors said.
The National Public Prosecutor's Office also said a case involving two other people who allegedly cooperated with the suspect had been referred to family and juvenile courts.
The ABW said Paweł G., like two people detained earlier, had been active in Telegram and Discord groups linked to neo-Nazi and extremist circles.
The agency said he had taken part in discussions about previous attacks and their perpetrators.
Some conversations also concerned potential attacks, with participants analysing the methods used by terrorist attackers and treating them as a source of inspiration, the ABW said.
According to the agency, Paweł G. belonged to a group called Atomwaffen Polska and helped create another group known as Bierut Soldier ZNB.
Both groups featured neo-Nazi content and symbols and included discussions about violence and attacks, the ABW said.
The agency said its investigation also identified another person linked to the circles in the central Łódź region and a minor German citizen.
According to information gathered by the ABW, the German minor had expressed a willingness to carry out an attack using a firearm motivated by racial hatred, xenophobia and religious intolerance.
The ABW said it passed that information to a foreign partner service, which initiated proceedings against the minor.
Jacek Dobrzyński, a spokesman for the minister coordinating Poland's intelligence services, said on X that Paweł G. was a 17-year-old from Siemianowice Śląskie in southern Poland.
Dobrzyński said the teenager was fascinated by Nazi ideology and had allegedly been preparing an attack in which many people were to die, including by planning to acquire weapons and military equipment.
He also said authorities were investigating whether anyone outside the extremist circles—including, he suggested, Russian services—could be influencing young people to commit such acts.
Dobrzyński linked the case to the detention last week of two 16-year-olds by the ABW and police in the southern Silesia region.
The agency said analysis of devices belonging to those teenagers had uncovered neo-Nazi, racist and antisemitic material, as well as content concerning terrorist attacks, mass killings and so-called lone shooters.
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Source: IAR/PAP