Nicole Alvarez received a subpoena to appear before the grand jury on Wednesday morning, said Joseph Low IV, adding that prosecutors declined to give him any more details
He also declined to characterize the relationship between Alvarez and Dr. Conrad Murray.
Newspaper reports have called her Murray's girlfriend and said the pair have an infant son together. Alvarez's apartment was searched last month.
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Murray is the target of an investigation into Jackson's June 25 death, which has been classified a homicide. He told police he gave the pop superstar the powerful anesthetic propofol and other drugs in the hours before Jackson's death.
Murray's attorney has said the doctor didn't give Jackson anything that "should have" killed him.
The Los Angeles County District Attorney's Office, which would prosecute the case against Murray if one is filed, declined to comment.
A grand jury subpoena is an effective way to force reluctant witnesses to testify, said Stan Goldman, a professor of criminal procedure at Loyola Law School Los Angeles.
"They can be held in contempt just like refusing in court," Goldman said.
He said it's hard to tell what Alvarez's testimony to grand jurors might mean for Murray.
It's possible authorities "don't have such a strong case that they want to do without this woman," Goldman said. "They'd rather have her.
"On the other hand, it's not good, because they're still interested it seems."