Authorities in the Gaza Strip should order a criminal investigation into the death of a man who died five days after Hamas security officials arrested him, Human Rights Watch said Tuesday.

Relatives of Adel Razeq, a 52-year-old father of nine, say that when security officials arrested him on April 14, they did not present a warrant and took him away under false pretenses.

When his brother examined the body, it was badly bruised and appeared to have broken bones, he says.

That, if true, would cast doubt on an Interior Ministry statement that Razeq died of an unspecified illness, said a statement from Human Rights Watch urging Hamas to "come clean" on the death.

"The evidence raises concern that governmental authorities may have caused or contributed to the death of a man in their custody," said Joe Stork, deputy Middle East director at Human Rights Watch.

"Hamas should order a fully independent criminal investigation."

Last Tuesday, an Interior Ministry statement said the prisoner suffered a "sudden illness" and was taken to hospital where he was pronounced dead. It did not mention why he was being held.

The Ministry of Interior has claimed that Rezeq drank chlorine in the detention center. According to the statement, an investigation was opened at the time into the cause of Adel Razeq's death.

He was taken to the Ash-Shifa Hospital for medical observation and afterward was returned to the detention center, with Rezeq dying due to medical complications, it said. However, the statement gave no further details on the circumstances of his death.