India has charged 33 men aboard an armed ship operated by a U.S. maritime security firm for failing to produce papers authorising it to carry weapons in Indian waters.
The captain and the chief engineer were not among those arrested in Friday's action.
The crew have been charged with illegal procurement of diesel and possession of arms and ammunitions without required documentation.
"The captain kept saying that he would produce the required documentation, but whatever was produced was inadequate," a police officer, who did not wish to be identified, told Reuters from the southern city of Chennai.
Police are still checking the authenticity of the documents on the ship, Chacko Thomas, a spokesman in India for Virginia-based AdvanFort, which owns the Sierra Leone-flagged ship Seaman Guard Ohio, told Reuters.
India detained the ship last week and it was being held in the port of Tuticorin along with its crew and armed security guards, which included British, Estonian, Indian and Ukrainian nationals.
The U.S. embassy in New Delhi could not be immediately reached for comment.
The southern tip of India is close to major trading routes from Asia to Europe. Many cargo ships now travel with armed guards to deter pirates. Sri Lanka, close to Tuticorin, is a popular boarding point for private armed guards.
A diplomatic row erupted last year when two Indian fishermen were allegedly shot dead by two Italian marines serving as security guards on an Italian-flagged oil tanker off the Kerala coast. The marines are currently being prosecuted in India.
The incident highlighted the loosely-regulated practice of placing private and military armed guards on ships for protection against pirate attacks.
Pirate attacks cost billions of dollars every year - as much as $5.7-6.1 billion in 2012, according to The Oceans Beyond Piracy advocacy group.