On July 3, the magistrate acquitted 12 members of the police Special Task Force (STF) and a police officer due to a "lack of evidence".
The global notoriety of the "Trinco Five" killings had made it a test case for the commitment of successive Sri Lankan governments to ensure accountability for grave crimes carried out during the three-decade-long civil war, Human Rights Watch and Amnesty International said.
"Sri Lankan authorities have proven unable to obtain justice for the murders of five young people and the resulting cover-up despite the considerable evidence available," said Human Rights Watch.
"The failure to convict anyone in this emblematic case after 13 years demonstrates the need for a court with international participation that can properly protect victims and witnesses," it added.
On January 2, 2006, amid New Year celebrations at Trincomalee beach, Sri Lankan security forces shot and killed the five students and seriously wounded two others.
The government quickly claimed, without evidence, that the youths killed were Tamil Tiger insurgents.
"The acquittals in the 'Trinco Five' case mean that the government's obligation to bring to justice those responsible for the murders remains," said Amnesty International.
The prosecution of the case had long been marred by serious threats against the survivors and witnesses which forced them and their family members to flee Sri Lanka.
( With inputs from IANS )