Fire officials say the blaze broke out around 5:00 a.m. at a home in the unit block of Saratoga Court in the Valley Wood development. Crews were met with heavy fire, which was tearing through the home.
Two adults, a mother and her son, got out safely, but the family dog perished in the blaze.
Damage is estimated at $200,000.
Investigators say it happened about the same time another fire started at the same home two weeks ago.
State investigators picked through the debris this morning. Samples were taken from the front of the home and placed into silver canisters. An arson dog was brought in to the property and investigators went door to door taking statements from neighbors.
At the end of the day, investigators determined the cause to be arson.
Anyone with information is urged to call The New Castle Division of The State Fire Marshal at 302-323-5375 or Crimestoppers at 1-800-TIP-3333.