Shaima Alawadi, 32, died several days after being brutally beaten inside her El Cajon home in March.
Police said a similar note was left outside the family home earlier in the month, but the family did not report it.
"A week ago they left a letter saying, 'This is our country, not yours, you terrorists,'" the daughter, Fatima Alhimidi told KGTV.
At the time, authorities did not rule out the possibility her death could have been a hate crime, though they said then they were examining other possibilities as well.
"After months of hard work, we determined that this homicide was a result of domestic violence," El Cajon Police Chief Jim Redman said during a press conference on Friday.
According to court records, Alawadi planned to divorce her husband and move to Texas to be with relatives.
On Thursday, her husband Kassim Alhimidi, 48, was arrested on suspicion of murder. He is being held without bail and is due in court on Tuesday.
Alawadi's death drew international attention.
Iraqi Prime Minister Nouri al-Maliki provided a plane that flew Alawadi's body back to Iraq, where the mother of five was buried.
Dozens of people attended the funeral, including her husband.
Alawadi and Al-Himidi moved to the San Diego County area in 1995 and then briefly lived in Dearborn, Michigan, before moving back to California.
El Cajon, where the family lived, and Dearborn have sizable Iraqi communities -- among the nation's largest.
U.S. State Department spokeswoman Victoria Nuland called the attack on Alawadi an "absolutely brutal beating."
She said in March that "authorities are continuing to search for motives behind this attack, but the United States has no tolerance for wanton acts of violence like this."