PARIS • A suspected Islamic State in Iraq and Syria (ISIS) operative who was arrested last week had amassed a trove of guns and bomb-making equipment, including the type of explosive used in terrorist attacks in Paris and Brussels, the French authorities announced on Wednesday, reinforcing fears that militants are planning additional assaults on Europe.

Frenchman Reda Kriket, 34, was arrested on Thursday afternoon last week in Boulogne-Billancourt, a western suburb of Paris.

That evening, a raid on the fourth-floor apartment he rented under a fake name in Argenteuil, a north-western suburb, yielded "an arsenal of weapons and explosives of an unprecedented size". It led the authorities to believe Kriket had been planning an "imminent attack", Paris prosecutor Francois Molins said at a news conference on Wednesday.

The arsenal included explosive materials - among them triacetone triperoxide (TATP), which was used in suicide bombs that were set off in Paris on Nov 13 last year and in Brussels on March 22 - along with Kalashnikov assault rifles, a submachine gun, pistols, ammunition, four boxes containing thousands of small steel balls, stolen French passports, brand-new cellphones, a tear-gas canister and two computers with instructions on how to make explosives.

Kriket was charged on Wednesday with terrorist conspiracy, possession of weapons and explosives, and falsification of documents, among other offences, Mr Molins said. He had an extensive criminal record, with multiple convictions for robbery, possession of stolen goods and acts of violence. More recently, according to court records, Kriket raised money for a network of militants in 2012 and 2013.

He and an accomplice, Anis Bahri, 32, are believed to have visited Syria between late 2014 and early last year, Mr Molins said. After returning to Europe, they travelled back and forth to France, Belgium and the Netherlands.

On July 29 last year, a Belgian court convicted Kriket in absentia of financing an Islamist recruitment network with proceeds from robberies and other crimes. Abdel- hamid Abaaoud, believed to have been the chief planner of the Nov 13 Paris attacks, was named in the proceeding.

Mr Molins said investigators had not uncovered a specific target or date for the planned attack and were trying to identify other accomplices. Under questioning, Kriket denied being a terrorist, saying he had rented the Argenteuil apartment on behalf of someone he would not name. He said the unidentified man and an accomplice were the bombmakers.

Three men have been held in other countries on suspicion of working with Kriket.

One is Bahri, who was arrested by Dutch police in Rotterdam on Sunday. The other two are Algerians who were arrested in Brussels last Friday: Abderahmane Ameroud, 38, and a 34-year-old man identified only as Rabah M.

A government official also revealed on Wednesday that a file with the floor plan and photos of the office of the Belgian prime minister had been found on a laptop discarded in a dustbin last week by one of the terrorists linked to the Brussels attacks. It was found during a raid in the Schaerbeek neighbourhood hours after the attacks.

On the same computer, investigators found a statement, described by the authorities as a will, by Ibrahim El Bakraoui, one of three suicide bombers in the attacks.

The attacks killed 32 people, a lower toll than previously reported, having been revised downwards on Tuesday as victim identification was completed.

NEW YORK TIMES