By Serajul Quadir and Douglas Busvine

DHAKA/NEW DELHI (Reuters) - A man who was celebrating his daughter's 13th birthday with his family in a Dhaka cafe when they were caught up in a bloody hostage-taking nearly four weeks ago is still being held as a suspect, the family's lawyer said on Wednesday.

Hasnat Karim, a 47-year-old engineer who holds dual British and Bangladeshi citizenship, survived the ordeal in which 22 hostages died at the hands of Islamist militants because he and his family were Muslims and could recite verses from the Koran.

But, after witnesses said he agreed to a demand to act as a human shield, Karim became a suspect in the July 1 attack claimed by Syria-based militant group Islamic State.

His wife and their two children were released after two days of questioning, but Karim continues to be held at an unknown location without access to his lawyers, according to Rodney Dixon, hired as international counsel by the family on Wednesday.

"He was there for his daughter's birthday, and got caught up as a hostage himself, together with his family," Dixon told Reuters in a telephone interview from London. "No evidence has been pointed to, and there is none which would show that he was in any way involved in committing any crime."

Human rights groups including Amnesty International have called on Bangladesh to either charge or release Karim.

In one of the most brazen attacks in the South Asian nation's history, gunmen stormed the Holey Artisan Bakery, a cafe in Dhaka's diplomatic quarter that was packed on a Friday evening with locals and expatriates.

They killed non-Muslims and foreigners - including Italians, Japanese and an American - before security forces ended the 12-hour siege. Karim made a phone call at the behest of the gunmen to a relative during the attack, and he his family were released before security forces stormed the building, giving rise to suspicions that he was complicit in the attack.

INVESTIGATION CONTINUES

Home Minister Asaduzzaman Khan told reporters on Tuesday that the investigation into Karim was continuing and until it is completed and police are satisfied he is innocent, he would not be released.

"We will not victimize any innocent people and we will not also spare anyone, whoever (they) may be, if he or she is found guilty," Khan said.

Karim had earlier taught at Dhaka's private North South University, where one of the five assailants had studied and where four other faculty members had been fired over links to a local militant group called Hizb-ut Tahrir.

Yet the university said Karim had not been suspected of links to any militant group. His family says he left to work in his father's engineering business.

Karim lived in Britain between the ages of 11 and 32, studying engineering at London's Queen Mary University before pursuing further studies in the United States and then returning home.

Lawyer Dixon said Karim was "not political at all" and had no links to radical Islamist groups.

"(His family) have all said this was the worst time of their entire life – and on top of that to have Hasnat arrested now for being involved in all of this is something which is impossible for them to comprehend," said Dixon.

Britain's Foreign and Commonwealth Office said its staff in Dhaka was in contact with the authorities following the detention of a Bangladeshi-British citizen and had requested consular access.

"We cannot interfere with the legal system of another country, just as other countries cannot interfere with the justice system in the UK," an FCO spokeswoman said.

Dixon said he would also file a petition to the United Nations Human Rights Commission in Geneva to try and get Karim's detention declared unlawful and for steps to be taken to secure his release.

(Additional reporting by Estelle Shirbon in London; Writing by Douglas Busvine Editing by Jeremy Gaunt)