* Gunman's wife could be charged as soon as Wednesday
* Senator says Noor Salman is "person of interest" in probe
* Obama calls U.S.-born killer "angry, disturbed, unstable"
* Orlando mayor to open assistance center for victims (Updates with details on family center for victims; edits)
By Letitia Stein and Julia Edwards
ORLANDO, Fla./WASHINGTON, June 15 (Reuters) - The wife of the gunman who killed 49 people at an Orlando gay nightclub could face criminal charges as early as Wednesday after a federal grand jury was convened to study possible wrongdoing by her, a law enforcement source said.
Omar Mateen's wife, Noor Salman, knew of his plans for what became the deadliest mass shooting in modern U.S. history, said the law enforcement source, who has been briefed on the matter.
U.S. Senator Angus King, a member of the Senate Intelligence Committee, which received a briefing on the investigation into Sunday's massacre, told CNN it appeared Salman had "some knowledge" of what was going on.
"She definitely is, I guess you would say, a person of interest right now and appears to be cooperating and can provide us with some important information," King said.
Salman was with Mateen when he cased possible targets in the past two months, including the Disney World resort in April, a shopping complex called Disney Springs and the Pulse nightclub in early June, CNN and NBC reported.
Orlando Mayor Buddy Dyer was due to open a family assistance center at a stadium on Wednesday where those directly affected by the tragedy will be able to get information, support and other resources, city officials said.
Mateen, who was shot dead by police SWAT team members after a three-hour standoff, called 911 during his rampage to profess allegiance to various militant Islamist groups.
Federal investigators have said he was likely self-radicalized and there was no evidence he received any help or instructions from outside groups such as Islamic State. Mateen, 29, was a U.S. citizen, born in New York of Afghan immigrant parents and worked as a security guard.
"He appears to have been an angry, disturbed, unstable young man who became radicalized," President Barack Obama told reporters.
Salman's mother, Ekbal Zahi Salman, lives in a middle-class neighborhood in Rodeo, California, about 25 miles (40 km) north of San Francisco. A neighbor said Noor Salman only visited her mother once after she married Mateen.
Noor Salman's mother "didn't like him very much. He didn't allow her (Noor) to come here," said neighbor Rajinder Chahal. He said he had spoken to Noor Salman's mother after the Orlando attack and she "was crying, weeping."
Mateen's rampage at Pulse was systematic as he worked his way through the packed club shooting people who were already down, apparently to ensure they were killed, said Angel Colon, a wounded survivor.
"I look over and he shoots the girl next to me and I was just there laying down and thinking, 'I'm next, I'm dead,'" he said.
Mateen shot him twice more, one bullet seemingly aimed for Colon's head striking his hand, and another hitting his hip, Colon said at Orlando Regional Medical Center, where he is one of 27 survivors being treated.
Vigils continued on Tuesday in Orlando. Hundreds of students gathered to pray and sing in the evening at the University of Central Florida. They shone cellphone flashlights during a reading of the names of the dead including two alumni.
The shooting raised questions about how the United States should respond to violent extremists at home and abroad. The Federal Bureau of Investigation questioned Mateen in 2013 and 2014 for suspected ties to Islamist militants but was unable to verify that he posed a threat.
Obama slammed Republican presidential candidate Donald Trump for his proposed ban on Muslims entering the United States, joining fellow Democrat Hillary Clinton in portraying Trump as unfit for the White House.
Trump criticized Obama for not using the term "radical Islamic terrorism" to describe Islamist militants. Obama replied that using that label would not accomplish anything.
Mateen made 911 calls from the club in which he pledged loyalty to the leader of Islamic State, Abu Bakr al-Baghdadi, whose organization controls parts of Iraq and Syria.
"We could hear him talking to 911 saying that the reason why he's doing this is because he wants America to stop bombing his country," said Patience Carter, 20, who was trapped in a bathroom stall at the nightclub as Mateen prowled outside.
U.S. officials were investigating media reports that Mateen may have been gay but not openly so, and questioning whether that could have driven his attack, according to two people who have been briefed on the investigation and requested anonymity to discuss it.
Barbara Poma, the owner of the Pulse nightclub, speaking through a representative, denied reports Mateen had been a regular patron.
"Untrue and totally ridiculous," spokeswoman Sara Brady said in an email when asked about the claim.
A former wife of Mateen, Sitora Yusufiy, said her ex-husband had facets of his life that he did not share with his family, such as drinking and going to nightclubs.
"He did have a different side to him that he could not open up to his father about," Yusufiy told CNN. "It doesn't surprise me that he might be gay."
She has previously said he was mentally unstable and beat her and that she fled their home after four months of marriage.
Additional reporting by Eric Beech in Washington, Barbara Liston, Bernie Woodall and Yara Bayoumy in Orlando, Brendan O'Brien in Milwaukee, Wis., Zachary Fagenson in Port St. Lucie, Fla., and Alexandria Sage in Rodeo, Calif.; Writing by Alistair Bell and Fiona Ortiz; Editing by Dominic Evans and Bill Trott