The AP reports: 

At least six people were killed Sunday when a gunman opened fire at a Sikh temple near Milwaukee, and the suspected shooter later died in an exchange of gunfire with police, authorities said. Police were called to the Sikh Temple of Wisconsin in the suburb of Oak Creek, where witnesses said several dozen people were gathering for a service. Authorities found four people dead inside the temple and two outside, Greenfield Police Chief Bradley Wentlandt said.

President Barack Obama was "notified of the Wisconsin shooting shortly before 1 p.m. by his Homeland Security Advisor, John Brennan," a White House official told the pool reporter. "The President continues to receive updates on the incident."

UPDATE: Obama responds:

"Michelle and I were deeply saddened to learn of the shooting that tragically took so many lives in Wisconsin. At this difficult time, the people of Oak Creek must know that the American people have them in our thoughts and prayers, and our hearts go out to the families and friends of those who were killed and wounded. My Administration will provide whatever support is necessary to the officials who are responding to this tragic shooting and moving forward with an investigation. As we mourn this loss which took place at a house of worship, we are reminded how much our country has been enriched by Sikhs, who are a part of our broader American family."

UPDATE II: More details from the White House:

At 4:30PM EDT, the President convened a call with FBI Director Bob Mueller, Chief of Staff Jack Lew, and Homeland Security Advisor John Brennan to receive an update on the tragic shooting in Oak Creek, Wisconsin. Following that briefing, the President called Wisconsin 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.  During the briefing, the President was informed that the situation at the Sikh Temple was under control and that the lone gunmen was killed by an Oak Creek police officer.  The President also was updated on the condition of some of the victims of the attack, and he directed that the federal government assist as appropriate in the investigation into the shooting.  The President said that he wanted to make sure that as we denounce this senseless act of violence we also underscore how much our country has been enriched by our Sikh Community, who are an integral part of our broader American family.