India said a Pakistan-based Islamic militant group claimed responsibility Tuesday for an attack on a U.S. government cultural center, which killed four Indian police officers.

No Americans were injured in the attack, but eighteen police officers, one pedestrian and a private security guard were wounded. State Home Secretary Amit Kiran Deb said the building was closed for the day.

In the attack, four people draped in shawls rode two motorcycles up to the American Center in the heavily populated center of Calcutta at 6:30 in the morning, shot at officers and fled.

A pool of blood could be seen outside the American Center. Spent AK-47 bullets littered the street.

The attackers were not hurt, said Calcutta Police Commissioner Sujoy Chakraborty.

"They used high-caliber automatic weapons. It is a terrorist attack but we cannot yet identify which group was involved," Chakraborty said.

He said the attack occurred when the police officers on the night shift were changing guard.

Armed forces were quickly deployed to guard bus and railway stations, bridges and other key facilities throughout the state.

In the Indian capital of New Delhi, a security cabinet held an emergency meeting and a high alert was issued at the U.S. Embassy and other American institutions.

FBI Director Robert Mueller, who was in India meeting officials, suggested it was too soon to label the shooting a terrorist attack.

"I think I will describe it as a horrible tragedy and an assault on police officers," Mueller told reporters. "But categorizing it beyond that, in the absence of the facts ... would be inappropriate at this time."

Still, India's home minister, Lal Krishna Advani, called it a "terrorist attack" against U.S. and Indian interests.

A senior home ministry official told The Associated Press that a man called police in New Delhi and said the attack had been carried out by Harkat-ul Jehad-e-Islami. Advani said the Pakistan-based militant group, which fought the Indian army in Kashmir in the 1990s, is connected to Pakistan's spy agency, the Inter-Services Intelligence.

A spokesman for the group said India manufactured the claim.

"None of our members was behind it," Mohammed Rizwan Kashmiri said.

Pakistani Foreign Ministry spokesman Mohammed Aziz Khan also denied the claim.

"These are all baseless allegations. There is no truth that Pakistan's ISI was behind the attack on the American Center," he said.

In Calcutta, however, Deb said another call to police, traced to Dubai, also said Harkat claimed responsibility. One local Calcutta newspaper received a fax from an unidentified group saying, "We have taken over your city." The fax also threatened to blow up the Howrah Bridge in Calcutta, which spans the Ganges River.

Another newspaper in Calcutta received a phone call from an unknown group called Asif Reza Commando Force, which claimed responsibility for the attack.

Police had earlier said that both newspapers had received faxes from Harkat.

Foreign Ministry spokeswoman Nirupama Rao said a "mafia don" based in Dubai and named Farhan, alias Aftab Malik, had claimed responsibility for the attack.

"It is obviously a terrorist attack," she said. "Farhan could be in touch with Harkat-ul Jehad-e-Islami and Harkat-ul-Mujahedeen," another Pakistan-based militant group.

The Calcutta office of Prime Minister Atal Bihari Vajpayee's Bharatiya Janata Party said it also received a telephone call threatening to blow up the local office.

The United Jehad Council, a coalition of Kashmir-based Islamic militant groups, also denied any involvement and claimed India's spy agency was responsible.

Joint Commissioner of Police Suresh Roy said there were intelligence reports of a possible strike against U.S. establishments in India as part of an "unprecedented threat" during India's Republic Day celebrations this coming Saturday.

"We have tightened the entire security arrangement at the U.S. Embassy and all U.S. establishments. A general alert has been sounded," Roy said in New Delhi.

Security has been tightened in recent months at the American Embassy in New Delhi, consulate buildings and other offices across the country to prevent possible attacks by Islamic guerrillas fighting to separate the Himalayan region of Kashmir from Indian control.

The American Center, one of the two U.S. government buildings in Calcutta, houses a popular library, the embassy's public affairs office, a press section and a wing for cultural programs.

The center is one of the landmarks of Calcutta, the eastern metropolis that is the capital of the communist-run West Bengal state. Anti-American protests by communists and labor unions are often held there, but this is the first violent attack against an American compound in India.

The attack came a day after Indian and American officials held the first meetings of the Joint Working Group on Counterterrorism in the aftermath of the Sept. 11 terror attacks in the United States.

The attack on the American Center comes more than a month after an assault on the Indian Parliament left nine government workers and five attackers dead in New Delhi. India blamed two Pakistan-based Islamic militant groups for the attack, which prompted the threat of war between the nuclear neighbors.

The United States has sought to mediate the conflict to keep efforts in the region focused on the U.S.-led campaign against terrorism in Afghanistan.

The Associated Press contributed to this report.