LA PORTE, Ind. (CBS) — Three people and a dog are dead, and two others are critically injured, after a nasty crash on Tuesday involving three cars and a tractor-trailer.

Three passengers and a Rottweiler puppy from Niles were in a Hyundai going westbound on US 20, when police say a Buick going eastbound crossed the center line and slammed into the car. The crash set off a chain reaction, involving an Infiniti and a semi.

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Jeffery Williams, 33, and Catrina Bontrager, 30, were in the Buick. They were pronounced dead at the scene.

Issa Sweilem, 27, and Joseph Ghandour, 26, were in the Hyundai and are both at South Bend Memorial hospital in critical condition. Michael Nahhas, 23, and the puppy were pronounced dead at the scene.

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The drivers of the Infiniti and the semi, Mark Moore, 48, and Ronald Rector, 61, both refused medical treatment at the scene.

The cause for the Buick traveling across the center line is unknown. Drug and alcohol testing came back negative.

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La Porte County Coroner Bob Cutler ruled massive blunt force trauma as the cause of death for all three victims.