Mexican authorities say they have detained seven more federal police officers in the northern border city of Matamoros in connection with an extortion investigation.

A statement issued late Thursday by the National Security Commission says the seven were taken into custody Thursday at the Matamoros airport.

On Saturday, Mexican officials said soldiers and marines had detained 15 other federal police officers in Matamoros for kidnapping a businessman and demanding a $2 million (31 million peso) ransom.

Thursday's statement did not say if the latest detentions were related. It said only that the seven officers were accused of extortion.

Matamoros lies across the Rio Grande from Brownsville, Texas. It continues to be gripped by violence in spite of an ongoing security operation with federal police and the military.