He screamed out his wife's name - "Ping! Ping!" - and then was shot in the chest and left arm.
Xiu Ping Chen, 44, said she ran out to find her husband lying near a fire hydrant on Sixth Street. He could not speak, but as she held him, he made a motion of a gun with his hand.
Chen said there were two white men nearby, watching as she pleaded for help in Chinese.
"They stood watching me crying. They didn't come to help us," she said. "They walked away."
At 7:20 a.m., Liu was pronounced dead at Thomas Jefferson University Hospital.
Qi Fang Liu, 22, was at the Massachusetts Institute of Technology when she got the call that her father was killed. She flew down that afternoon.
"Every day he drove my mother to work in the morning," she said in her parent's home yesterday as her mother wailed in sorrow.
Homicide Capt. Michael Costello said his detectives had no suspect or motive. Nothing was taken, although Costello did not discount that it may have been a botched robbery attempt.
That Liu screamed before he was shot indicates there may have been a struggle.
But Liu's daughter said her father regularly warned his family not to struggle if they were threatened by a robber.
"Always put down everything you have," she recalled his saying.
She said he did not have enemies and was careful to avoid conflict.
"I don't understand this," said the MIT senior, who is majoring in both math and management science.
Liu got married in 1983 and brought his wife to Philadelphia two years after he arrived here in 1992. He brought his daughter and son, Si Yuan Liu, now 21, in 1997.
His daughter said they don't know what they will do now.
"We don't want to live here anymore because it is so sad," she said. "But we really don't want to sell the house because dad built it."
Contact staff writer Robert Moran at 215-854-5983 or firstname.lastname@example.org.