(CNN) -- A man accused of killing a woman and her four children in Brooklyn pleaded not guilty to murder charges Monday, a spokeswoman for the Kings County District Attorney's Office said.
Mingdong Chen, 25, is being held without bail in the Saturday stabbing spree, spokeswoman Mia Goldberg said.
Chen made statements to police about his role in the deaths, New York police Chief Phillip Banks said. Chen, through an interpreter, also said he used a butcher's knife, which was retrieved from the apartment where the crime took place, Banks said.
Police are still trying to understand what could have set off Chen, who had been staying with the family for about a week before the stabbings in the Sunset Park neighborhood, Officer Sophia Tassy-Mason said.
"He made very soft comments that since he's been in this country, everyone seems to be doing better than him," Banks said.
Chen, who has no criminal record in New York City, has been charged with one count of first-degree murder and four counts of second-degree murder, as well as criminal possession of a weapon, Tassy-Mason said.
Chen also allegedly assaulted an officer who processed his arrest, which added the charges of assaulting a police officer and resisting arrest, Banks said.
Police identified the victims as Qiao Zhen Li, 37, whose husband is Chen's cousin; Linda Zhuo, 9; Amy Zhuo, 7; Kevin Zhuo, 5; and William Zhuo, 1. All five were stabbed in their upper bodies, police said. Three of the children died at the scene, and the other victims were pronounced dead at nearby hospitals, police said.
Chen's last known address is in Chicago, and he has moved a lot since immigrating to the United States from China in 2004, Banks said.
The mother of four was at home alone with her young children and Chen on Saturday when she noticed Chen acting suspiciously, Banks said. When she was unable to call her husband, who was at work, Li called her mother-in-law, who lives in China, he said. Li's mother-in-law alerted her daughter, who arrived at the home with her husband to check on the family. After banging on the door for some time, they told police Chen finally opened the door covered in blood, Banks said.
CNN's AnneClaire Stapleton and Allie Malloy contributed to this report.