York Regional police say a 20-year-old man remains in custody after four people were found dead in a Markham home on Sunday afternoon.

Police were called just before 3 p.m. to a house on Castlemore Avenue, just east of Mingay Avenue, for a report that multiple people were injured.

When officers arrived, they were met by the man at the front door.

Police said when they searched the residence, four people were found deceased inside.

Officers have not identified the man taken to custody and are calling the incident a "suspicious death investigation."

Const. Andy Pattenden told reporters that the bodies remain inside the home and the coroner, homicide detectives and the forensic services unit have begun their investigation.

"They've got a long job ahead of them," said Pattenden. "This is quite a scene inside this residence."

Pattenden would not say if the man in custody is related to the four people as their identities are yet to be confirmed.

A neighbour told CP24 said he was surprised by the incident, describing it as "unusual."

Pattenden said residents in the neighbourhood should expect the police to stay in the area for the next few days.

"We want to reassure everybody that York Regional police is out here in full force and that we have all our resources available to this investigation," Pattenden said.

He said the man in custody has not been charged.

Pattenden said they do not have any information about any altercation that occurred inside the residence.

"We do need the public's help," said Pattenden. "If anybody saw anything or anything heard here today or anything suspicious at all, contact our homicide unit."