The man who killed her was Kevin James Loibl, 27, of St. Petersburg, Florida, according to Orlando police.
But they didn't give any background on Loibl or offer a possible motive.
CNN affiliate WFTS photographed a handwritten note posted on Loibl's family home. It reads, "Deepest sorrow for lost (sic) to the family, friends and fans of the very talented, loving Christina Grimmie. No other comments."
Grimmie was signing autographs late Friday after a show at The Plaza Live theater when a man approached and opened fire, police spokeswoman Wanda Miglio said.
She was rushed to a hospital, where she died.
'More than my sister'
Loibl fatally shot himself after the singer's brother, Marcus Grimmie, tackled him, police said in a
statement. About 120 people were at the meet-and-greet, and the brother's quick action may have saved other lives, police said
Marcus Grimmie paid tribute to the singer in a Facebook post, saying he has no words.
"Christina was more than my sister. She was a partner in life," he posted. " A superstar. A goofball. Introverted. And a friend to everyone. Genuinely. But above all...she was my baby sister."
The shooter had two handguns, two additional loaded magazines and a large hunting knife, police Chief John Mina said.
Mina said investigators were going through the man's computer, phone and other belongings to see whether they can find information about a motive. There's no evidence Grimmie and Loibl knew each other, police said.
Police said the suspect drove to Orlando, apparently to commit the crime, and "then had plans to travel back to where he came from."
Theater suspends shows
It's unclear how the gunman got a gun into the venue or whether attendees were checked for weapons.
"We don't know if he was just a crazy fan that followed her on Twitter or on social media," Miglio said. "We really don't know ... it's undetermined at this point."
Security measures are in place at The Plaza Live, police say, but there are no metal detectors and people are not patted down.
"As far as security, The Plaza does have security guards who wear polo shirts. They are unarmed," Mina said. "They do check bags and purses as people enter."
The Plaza Live issued a statement extending condolences to the victims and indicating all activities and events have been suspended "until further notice."
"Our thoughts and prayers are with the families of those lost as well as those that witnessed and aided in the recovery after this senseless act," the statement said.
Grimmie posted a video hours before her last performance in which she jovially invited fans to see her show.
She was discovered on YouTube
, where she amassed more than 3 million followers. Grimmie went on to win a spot on Season 6 of NBC's "The Voice" after her show-stopping performance of Miley Cyrus' "Wrecking Ball" during a blind audition captivated the attention of all four contestant judges.