DALLAS — A third Texas man has pleaded guilty over what federal prosecutors call a more than $6 million diamond investment scam.

Craig Allen Otteson of McKinney pleaded guilty Tuesday to mail fraud. The 65-year-old Otteson appeared before a judge in Dallas over a scheme that investigators say ran from March 2011 to late 2013.

Jay Bruce Heimburger of Dallas last week pleaded guilty to mail fraud. Christopher Arnold Jiongo of Houston in May pleaded guilty to wire fraud.

All face up to 20-year prison terms.

Investigators say more than 75 investors were scammed out of $6.4 million linked to purchase and resale of diamonds on the international market. Prosecutors say money and property were fraudulently secured by the three men, who could be required to make restitution.