MEXICO CITY, March 11 (Xinhua) -- Mexican authorities said Sunday that an explosion on a passenger ferry last month was neither a terrorist attack nor carried out by an organized crime group.

On Feb. 21, 24 people including five Americans were injured in the blast aboard a ferry that shuttles between Playa del Carmen and Cozumel island, two of eastern Mexico's top tourism destinations.

The passengers were mainly tourists and employees of hotels, restaurants and other businesses.

Authorities are still investigating how and why the homemade "rudimentary explosive" was planted on the ferry, which was docked in Playa del Carmen when the device exploded and tore a hole into the side of the vessel.

Probes are still trying to identify and arrest the probable suspects, Alberto Elias, spokesperson for the Mexican Attorney General's Office, told reporters at a press conference.

The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives and the Federal Bureau of Investigation are aiding the investigation, he said.

Mexican officials have stepped up security at the ferry docks with additional officers aided by sniffer dogs.

Less than two weeks after the blast, a second explosive device was reportedly found aboard another vessel belonging to the same ferry operator, whose operations have been suspended by the government.

The U.S. embassy in Mexico has issued an alert and barred staff from taking the ferries.

The incidents have not disrupted daily life in the region, Mexico's Federal Police Commissioner Manelich Castilla said at the press conference.