He said it was remarkable that no information from those responsible for the shooting, or on its periphery, had ever come forward.
The coroner said the inquest at Belfast Coroners' Court may be the last opportunity to do so.
No-one has ever been convicted in connection with the killings.
The men were shot dead on 5 January 1976 after IRA gunmen stopped their van and asked which among them was a Catholic, and instructed that man to leave the scene
The main witness at Monday's hearing, retired police officer Charles Hamilton, told the inquest that the IRA attack was not expected.
He said if anyone had suggested it as a possibility he would have said they were wrong, because they were all innocent men and had no connection to the security forces.
The former detective identified a number of men suspected of being involved in the killings.  Rather than naming the suspects they were given code numbers instead.
The inquest into the killings resumed last week after being adjourned in 2016 due to a police investigation.
The court heard last Wednesday that a key suspect had been linked to almost 50 murders.
The coroner at the inquest into the murders of 10 Protestant workmen in Kingsmills, County Armagh, 41 years ago, has appealed for more witnesses.