The Federal Bureau of Investigation has joined a US state department probe into allegations that employees of Blackwater, the US private security company, shot and killed Iraqi civilians in ­Baghdad last month.

Nouri al-Maliki, the Iraqi prime minister, accused the company of committing a crime in the September 16 incident, which resulted in the deaths of at least 11 Iraqis. Blackwater insists its guards fired in self-defence after a convoy it was protecting came under attack.