CALCUTTA, India — Police arrested three Bangladeshis and three Indian teachers at an Islamic school and charged them in an attack on a U.S. cultural center that left four policemen dead, officials said Wednesday.

The Indian government appeared to back down from its earlier assertion that the shooting in Calcutta was a terrorist attack, saying it was too early to tell the motive, after the United States said it was not certain the attack was political.

India initially accused Islamic militants and Pakistan's spy agency after gunmen on motorcycles sprayed the American Center in Calcutta with gunfire on Tuesday, killing four policemen and wounding 20 people. Pakistan denied any role.

U.S. officials said Tuesday the attack may have been criminal, targeting the policemen. The Indian government on Wednesday was equivocal.

When asked if terrorists were responsible, Foreign Ministry spokeswoman Nirupama Rao replied: "This attack, the criminal incident yesterday, is still being investigated. We expect to have a more authoritative version from the evidence available."

The United States has been trying to defuse tensions between India and Pakistan, which have been on war footing with tens of thousands of troops at their borders since a December attack by gunmen on India's Parliament. India accused Pakistan's spy agency in that attack as well, a charge Islamabad denied.

Calcutta police superintendent Shivaji Ghosh said 55 people had been detained for questioning in addition to the six arrested for the American Center shooting.

The six have been charged with murder and waging war against the state, and face life imprisonment if convicted, he said.

The chief secretary of West Bengal state — where Calcutta is located — suggested police were still pursuing the terrorism motive.

"We are verifying their links with Pakistan's spy agency and a Dubai-based mafia don, but we can't disclose details at this stage," Sourin Roy said. Police have said a criminal boss based in the Persian Gulf emirate of Dubai may be involved.

Police said they also seized a motorcycle belonging to Korban Ali, one of the arrested religious teachers, and that it may have been used in the attack.

Police officials said a videotape taken by a closed-circuit TV camera at the entrance of the American Center had established there were two attackers riding on one motorcycle. Police had earlier believed there were four men riding on two motorbikes.

Officers who survived the attack said the gunmen, armed with assault rifles, appeared professional. The policemen, in contrast, carried outdated Enfield bolt-action rifles that jammed and, in some cases, were unloaded.

An Enfield "takes at least two and a half minutes to get loaded," Bablu Mondal told The Associated Press from his hospital bed Wednesday. "How can you expect us to face these gunmen armed with AK-47s?"

Indian officials have blamed the attack on Harkat-ul Jehad-e-Islami, a Pakistan-based group with a little-known branch in Bangladesh. The group has been fighting since 1994 to merge the Indian-held part of Kashmir with Pakistan. Harkat denied any involvement in Tuesday's attack.

Around 200 Harkat militants are believed to have entered Kashmir since the United States began military strikes against Afghanistan in October, said R.S. Bhullar, director-general of India's Border Security Force.

India accuses Pakistan's Inter-Services Intelligence spy agency of supporting the militants with arms and training. Pakistan denies giving Harkat and other militant groups in Kashmir material support, saying it gives only political backing to their cause.

Since the December attack on India's Parliament, Pakistan's president has said he will not allow terror attacks in the name of Kashmir by groups on his nation's soil, and Pakistani police have arrested hundreds of Islamic extremists.

The American Center in Calcutta, meanwhile, reopened Wednesday.

"We're open for business," said Rex Moser, director of the American Center. "There are new sandbags and armed commandos out front. I assume that would dampen attendance."