Prosecutors in Panama say an 18-year-old man has been charged following the killing of seven youths who were on a swimming trip to Gatún Lake.
The bodies were found in a wooded area near the lake on Saturday with gunshot wounds to their heads.
Their relatives had reported them missing when they failed to return from their excursion on Friday.
Six survivors said they had been threatened by two men who had led the victims away at gunpoint.
Who is the suspect?
Police have not named the suspect but said he had lived in the same area where the six victims were from.
He was arrested on Sunday in a mountainous area after the survivors had provided police with a description.
He has been charged with alleged homicide, femicide and robbery. His lawyer says that his client is innocent, did not match the description given by the survivors and did not know any of those killed.
A judge ordered he be held in detention. Police are still searching for a second suspect.
The survivors have told investigators that a group of 13 pupils and students set off for Lake Gatún on Friday morning.
They say that on their way back from the lake, they came across two armed men, one of them hooded, who stole their belongings, made threats and seized them.
One survivor said that one of the attackers appeared to know one of the victims, as he called out his name.
The seven victims were all from Valle Verde in Colón province, while the six survivors where not, which has led to speculation as to whether the attackers let those who were not from Valle Verde get away.
Those who managed to escape made their way to a police station to report what had happened.
The seven who could not get away – four young women and three young men aged between 17 and 22 – were found dead the following morning in woods in Espinar, an area described by neighbours as quiet and generally safe.
Five of the bodies were inside an old bunker which had been used by US soldiers when they were stationed in Panama. The bodies of two sisters lay outside the bunker.
All had gunshot wounds to the head and one body had three additional shots, local media reported.
Who are the victims?
Among the victims are two pairs of sisters. Azucena (18) and Yulissa Martínez (21), whose bodies were found outside the bunker, and Leydis Mabel and Yeleny Yesibel Marriota Villarreta, who were inside the bunker.
Betzaida Daley de Martínez told local media that her daughters Azucena and Yulissa had gone to the lake with their friends, whom she described as “upright youths” who were studious and some of whom played in a band.
Of the three male victims, two were university students while the other was still at school.
How violent is Panama?
Mass killings like the one by Gatún Lake are unusual in Panama.
Earlier this year the country was shocked by the deaths of six children and one adult during an exorcism carried out by religious sect in an indigenous area.
Gangs are active in Panama, but they are not as powerful or big as those in other Central American countries such as El Salvador.
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While the number of intentional homicides in Panama rose from 439 in 2018 to 479 in 2019, it remains below that of its southern neighbour Colombia and is just above that of its northern neighbour Costa Rica, according to figures collated by think tank Insight Crime.