But the pirates said they agreed to forgo a ransom after learning that Somali businessmen had hired the ship, which was taking oil from Djibouti to the Somali capital of Mogadishu.
Security official Ahmed Mohamed told Associated Press the pirates had disembarked the ship, which is now heading to the port of Bossaso, the region's commercial hub.
An unnamed former British army officer said that the Puntland maritime police force freed the ship after making an offer the pirates couldn't refuse.
A pirate said they chose to release the ship without any ransom payment, Reuters reported.
The firing had caused great concern in Sri Lanka especially after the trapped crew called up to ask the Sri Lankan government to do everything it could to stop it.
"We thank the USA government and the President of Puntland, the semi autonomous region of Somalia in whose waters the hijacking took place", De Silva said.
The tanker was forced to change course and head toward Puntland.
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The area where the pirates operate is also home to one of the eight biggest maritime oil routs, the Bab el-Mandeb channel, through which 3.8 million barrels of crude pass daily. The ship was anchored off the town of Alula, said Salad Nur, a local elder. They don't normally kill hostages unless they come under attack.
Piracy off Somalia's coast used to cost the world's shipping industry billions of dollars and was seen as a major threat.
"These are fishermen who are infuriated with the illegal fishing off their coasts".
Village elders of Alula said the pirates had not made clear demands, but claimed to be driven by anger over illegal fishing.
The vessel is owned by Panama-based company Comoros and managed by the United Arab Emirates, and was carrying a cargo of gas and fuel at the time of capture.
Though anti-piracy measures ended attacks on commercial vessels, fishing boats have continued to face attacks.