BRUSSELS—Belgian investigators found a martyr letter on a computer owned by one of the two men suspected of plotting an attack in Brussels during the holiday period, a person familiar with the matter said, as details emerge of the assault authorities say they foiled.

In the letter found on Mohammed Karay’s computer, the suspect said he was willing to die as a martyr and a good Muslim, but he didn’t spell out concrete details about a specific attack, the person said.

“The letter is typical of someone who wants to commit suicide in a terrorist attack,” the person added.

Authorities canceled traditional fireworks and other public festivities scheduled for New Year’s Eve after Belgian authorities charged two men with plotting to attack police and military personnel during end-of-year holiday celebrations in and around Brussels’ main square, the Grand Place. The two men charged were members of a motorcycle gang, Kamikaze Riders.

Some Belgians expressed skepticism about the plausibility of the supposed holiday attack given that authorities haven’t yet found guns or weapons in connection with the plot. Federal prosecutors have said authorities seized Islamic State propaganda material, cellphones, computer hardware and material from Airsoft, a game using replica guns.

Investigators are still determining the precise date when the letter was written, but they believe it was from last month, according to the person familiar.

Mr. Karay’s lawyer, Xavier Carette, said the family knew of the letter but denied that it was a suicide note. They said Mr. Karay wrote the letter with the aim of stating that he wanted his body buried next to his father’s grave in case he died in a motorbike accident, according to Mr. Carette.

“It’s a statement just in case… it’s not one in which he says he’s leaving for a just cause,” Mr. Carette said.

The family can’t publish the letter because it is on the computer seized by the authorities, said Mr. Carette, who said he hasn’t seen the letter himself.

Carine Couquelet, lawyer for Said Saouti, the accused ringleader of the suspected plot who is also charged with recruiting people into terrorism, has said Mr. Saouti appeared to be a radicalized Muslim even though her client has denied it.

The ISIS propaganda material was found in the homes of Mr. Saouti and Mr. Karay, according to the person familiar.

The lawyers for both of the men have denied all charges.

Two other people initially detained in the investigation, who also were members of the Kamikaze Riders biker group, told the investigating judge that there had been plans for an attack over the end-of-year holidays, according to the person familiar.

Prosecutors said the two had since been released but wouldn’t say why they had been let go.

Belgian authorities have said there was as yet no established connection between the Brussels holiday plot and the attacks in Paris on Nov. 13 but that investigators would continue to look for any links.

In connection with the Paris attacks, authorities on Tuesday prolonged by a month the detention of a Belgian man arrested in a raid at the end of December. The man, born in 1993 and identified only as Ayoub B., is the 10th person Belgian authorities have charged in connection with the Paris attacks.