MEXICO CITY (Reuters) - A blast last month on a tourist ferry in the Mexican beach resort of Playa del Carmen was caused by “an explosive device” that injured passengers, including U.S. tourists, the U.S. embassy in Mexico said on Thursday.

A security alert posted on the embassy’s website warned American citizens to exercise caution and said Mexican authorities continued to investigate the blast.

Two more explosive devices were found attached to another ferry on the island of Cozumel on Thursday, local media reported, saying marine troops had secured the scene.

The Ministry of the Marina did not immediately respond to a request for comment. The Quintana Roo state prosecutors office said federal agents were investigating the cases.

The federal attorney general’s office did not immediately respond to a request for comment.

The Quintana Roo state port authority said on Feb. 21 that the initial signs were that a mechanical failure had caused the explosion in Playa del Carmen that injured at least 18 people.

Local media reported up to 25 people had been hurt, but none of the injuries were life-threatening. Local media also reported that a drug gang had taken credit for the blast.

The same company, Barcos Caribe, operated both the ferry in Playa de Carmen and Cozumel, and is owned by the father of former Quintana Roo state Governor Roberto Borge, who is facing corruption charges, local media reported.

Barcos Caribe did not immediately respond to an email seeking comment.

Borge, from Mexico’s ruling Institutional Revolutionary Party (PRI), was extradited in January from Panama to face charges of embezzlement and abuse of public office. Borge has denied the allegations.