A Molotov cocktail exploded under the van of an instructor at the Tomlinson Adult Learning Center, creating a small fire but leaving little more than a black tinge on the ground and the back of the van. Eugene Hall, who teaches behavioral science and social studies at Tomlinson, said he believes the device might have been thrown by a disgruntled student. He said a student he reprimanded two weeks ago had threatened to get back at him. "Right now, it's a whodunnit," said Detective William Korinek, who assists the Fire Department with arson investigations. Korinek said the bomb was made with a glass bottle, a flammable liquid and a rag. No one was seen nearby when the bomb exploded.