Washington:The FBI will investigate Sunday's attack on a Sikh gurdwara in a Milwaukee suburb in which a gunman killed six worshipers and injured three others before being shot dead as an act of domestic terrorism.

The shooting at the gurdwara in Oak Creek at about 10.30 a.m. Sunday in which the first officer to respond to the scene was also injured is being treated as a "domestic terrorist-type incident", the town's Police Chief, John Edwards, said.

Oak Creek police officers who responded to a 911 call about the shooting were helping a victim when the shooter ambushed one of the officers, shooting the officer multiple times. A second Oak Creek officer returned fire, killing the shooter, Edwards said.

All three of the wounded were in critical condition at Milwaukee's Froedtert Hospital, spokesperson Carolyn Bellin told CNN. The congregation's president was among the wounded, his nephew Kanwardeep Singh Kaleka said.

Police did not release information about the gunman, with Edwards saying, "That is being checked into and is part of the criminal investigation." Nor would he disclose specifics of why the attack was being classified as an act of domestic terrorism.

The attack occurred about 10.30 a.m., when temple members were reading scriptures and cooking food in preparation for the main Sunday service and community lunch, Kaleka said.

Kaleka, who helped police interview witnesses, said members described the attacker as a bald, white man, dressed in a white T-shirt and black pants and with a 9/11 tattoo on one arm -- which "implies to me that there's some level of hate crime there".

A law enforcement official confirmed to CNN the shooter was wearing a white T-shirt and did not have a bulletproof vest.

CNN cited Teresa Carlson, special agent in charge at the FBI's Milwaukee division, as saying late Sunday that while authorities are looking into whether this "might be an act of domestic terrorism, no motive has been determined at this time."

White House officials said President Barack Obama was notified of the shootings shortly before 1 p.m. by John Brennan, his Homeland Security adviser. The president continues to receive updates.

After receiving a briefing at 4.30 p.m. from Brennan, FBI Director Bob Mueller, Chief of Staff Jack Lew, Obama called Governor Scott Walker, Oak Creek Mayor Steve Scaffidi and trustee of the Sikh Temple Charanjeet Singh to express his condolences for the lives lost and his concern for those who were injured.

Police searched the suspect's home, "a short distance" from the temple, Sunday evening, according to a law enforcement source cited by CNN. A single semi-automatic pistol believed to have been used by the gunman was found at the scene, along with the wounded officer's weapon, the source said.

Initial reports were that there may have been multiple attackers, but police found no indication of another gunman, said Bradley Wentlandt, the police chief in nearby Greenfield.

Congregation president Satwant Kaleka was shot and wounded when he attempted to tackle the gunman, his son, Amardeep Kaleka told local TV station WTMJ. His mother -- who hid in a closet during the violence -- was too distraught to talk, he said.