Indian police on Friday questioned 33 people aboard a ship operated by a US anti-piracy firm for carrying guns and ammunition in Indian waters without proper permits, reports said.
India's coastguard has said it stopped and detained the ship after discovering the cache of weapons and ammunition before escorting it on Saturday to the southern port of Tuticorin.
Police launched an investigation into the 10 crew and 25 armed security guards of the Seaman Guard Ohio which is registered in Sierra Leone and belongs to the US-based maritime security firm AdvanFort.
The 35, who include British, Estonian, Ukrainian and Indian nationals, were detained over the cache of some 30 assault rifles and around 4,000 rounds of ammunition, according to the Times of India website.
Thirty-three of them were taken to a local police station for questioning, while two remained on board, it said, citing unnamed police sources.
The crew had not maintained a log of the arms on board, according to NDTV, also using unnamed sources.
The incident comes after a furore over the shooting deaths of two Indian fishermen allegedly by two Italian marines off the coast of the southwestern Indian state of Kerala in February last year.
The southern tip of India is close to major trading routes from Asia and Europe and some cargo ships now travel with armed guards to deter pirates.
AdvanFort has said its guards regularly provided "armed counter-piracy protection" to cargo ships and therefore their weapons and amunition were stowed on board the Seaman Guard Ohio.
The company said all of the weapons were "properly registered and licenced to AdvanFort."
AdvanFort president William Watson played down the seizure of the ship, and instead thanked the Indian coast guard and police for allowing the ship to refuel and escape a cyclone that hit India's eastern coast on the weekend.
"I want to personally thank the Indian government for offering a safe harbor during this typhoon to the crew of our good vessel OHIO," Watson said in a statement this week.
"We look forward to returning this vessel to its duties as quickly as formalities and resupply operations are concluded."
India's Deputy National Security Advisor Nehchal Sandhu said on Thursday that the police investigation would be dropped if it was determined the ship had been outside of Indian waters.
"You have to understand that our territorial waters extend up to 12 nautical miles," Sandhu was quoted by the Press Trust of India saying.
"Anything that happens beyond that is not within the realm of our control."