Quebec's Independent Investigations Bureau (BEI) is investigating after a 23-year-old man died while in custody at the municipal courthouse in Montreal.

The man was to appear in court this morning after being arrested by Montreal police last night.

In a statement, the BEI said the man was placed in cell with about a dozen other people at around 7:30 this morning.

Just before 9:00, guards noticed the man unconscious on the floor of the cell.

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The Bureau of Independent Investigations (BEI) is charged with investigating incidents in which people die or are hurt while in custody or during a police operation.

The man was rushed to hospital where he was pronounced dead.

The bureau said investigators are questioning the other prisoners who were in the cell with the man.

The BEI is mandated to investigate incidents in which people die or are seriously injured while in custody or while interacting with police.