By Krisztina Than

SZEGED, Hungary (Reuters) - Hungarian prosecutors have opened an investigation into whether police can be held criminally responsible for abuse over the death of a Syrian migrant who drowned in the Tisza river as he was trying to cross into Hungary from Serbia this month.

Farhan al-Hwaish, 22, drowned in a branch of the river on June 1 when a group of migrants tried to cross into Hungary with the help of human traffickers. Police found his body two days later.

His brother, who crossed with him, says he drowned after Hungarian police guarding the border on the river bank threw objects at them, sprayed them with gas and unleashed attack dogs to prevent them from climbing out of the water.

The police deny any such mistreatment.

"This allegation is entirely unfounded," Hungary's Csonograd county police force said in an emailed response to Reuters on Tuesday, which said police rescued an Iraqi family of five from the river that day.

"Police conduct duties in a lawful, professional and proportionate manner, paying special attention to a humane treatment towards illegal immigrants, respecting their human dignity."

It noted that an autopsy on the body had concluded the death was caused by drowning, and that the body showed no injuries.

Hungary's prosecutors' office said in an emailed reply to questions from Reuters that it had ordered an investigation five days after the incident. The case would look into "suspicion of ill-treatment committed during an official procedure" by an "unknown perpetrator", it said, giving no identification of any officers involved.

"During the investigation we will examine what happened, and whether any of the police who were on duty on the given stretch of the border can be held criminally responsible," Andrea Nagy, a spokeswoman for the prosecutor's office said.

The incident is believed to be the first in which Hungarian police have been investigated over allegations they badly mistreated migrants at the EU's external frontier. The UN High Commissioner for Refugees has called for an investigation. [L8N18Y2G2]

Hungary's southern border was the main entry point for hundreds of thousands of Syrian refugees and other migrants traveling over land across the Balkans into the EU's border-free Schengen zone, until the Hungarian authorities abruptly shut the frontier last year.

Hungary swiftly built a razor wire fence and fortified it with police and army patrols. Its hard line at the border has since drawn criticism from rights groups, but Hungary says the law requires it to prevent unauthorized entry into the Schengen zone, and that most refugees or other migrants are safe in Serbia.

GO BACK! SWIM! SWIM!

According to the initial account of the incident reported on the national police website, a river patrol spotted two people at 5:55 a.m. on June 1 swimming across a branch of the river that forms the border with Serbia near the village of Roszke. The swimmers turned back towards Serbia after they were spotted, but only one made it.

The Hungarian police immediately notified the Serbian authorities and water rescue teams, and launched a search, rescuing the Iraqi family of five from the river at 6:30 a.m. and taking them to hospital, that account said.

Hwaish was finally buried in a cemetery in the Hungarian town of Szeged near the Serbian border on Monday, 17 days after police fished his body out of the river.

His younger brother, Abdullah al-Hwaish, told Reuters after the burial that nine migrants had tried to cross with the help of a smuggler. The group included an Iraqi family of five and four young men, including the Hwaish brothers from Syria and their younger cousin.

When they reached the Hungarian side, they climbed out of the boat into water up to their chests. The smuggler pulled the boat back to the other side on a string, he said.

According to his account, which Reuters was unable to verify independently, when the migrants tried to climb the bank of the river, police threw objects at them and sprayed them with a gas which caused his brother to choke and cough. He did not specify the type of gas.

"Every time we tried to get out they obstructed us. During the final attempt, they released police dogs on us," he said via an interpreter.

"The final attempt, when they wanted to release the police dogs, we begged them. They sprayed us with gas and said: 'This is oxygen,'" he said. "They just told us: go back (to) Serbia. Go back! Swim! Swim!"

By this time, gas was drifting in the air above the water, he said, adding that his brother was a good swimmer but disappeared.

(Reporting by Krisztina Than; Editing by Peter Graff)