A man and a woman in their twenties have been found dead inside a house.
Lincolnshire Police officers were called to an address on Tiber Road, North Hykeham, around 6.15pm yesterday, where they found the pair.
They were both pronounced dead at the scene.
A force spokesman said the causes of their deaths were not yet clear but officers were treating the incident as suspicious.
He said: "A post-mortem by a Home Office pathologist is due to be carried out this evening to establish the causes of death.
"Detective Superintendent Guy Collings, who is dealing with the case, will not be making any statement until tomorrow when he knows the causes of the deaths."
He added that a police cordon has been put in place around the property while officers carry out investigations and a forensic examination of the house takes place.
Anyone with any information is asked to contact Lincolnshire Police on 0300 111 0300.
A spokesman for the force said fire investigation officers had left the scene for the day but the inquiry was still ongoing.
Formal identification of the man and child has yet to take place.