A man was arrested early Thursday in connection with the fatal shootings of an infant and three adults in suburban St. Louis (search), police said.

Police found the bodies after neighbors called police Wednesday evening to report a burglary in progress.

Witnesses said someone had entered the basement apartment through a broken window, said Cmdr. Norman Venable of the St. Louis Major Case Squad (search). Police found 12-gauge shotgun casings inside the apartment and outside the building.

The victims were Jason Smith, 2 months, Nicole Willyard (search), 19, Mary Cawvey, 19, and Brandon Lobell, 23, Venable said.

The women and the baby lived in the apartment, but it was unclear which of them was the infant's mother, Venable said. The man who was arrested had a past "boyfriend-girlfriend relationship" with one of the women who was killed and is considered a suspect in the deaths, he said.