biochemistry at Leeds University and attended a graduate program in chemical engineering in North Carolina, according to counterterrorism officials. It is not clear whether Mr. Nashar is being sought as a witness or a suspect, but he is believed to have known some of the bombers. Also on Thursday, a new theory emerged about the nature of the explosives in the bombings. A senior American official and two senior European officials helping in the inquiry said investigators had discovered evidence of a low-grade, homemade explosive material in the case. One of those officials said the discovery had been made in the bathtub of a home that was raided, suggesting the material was made there. Earlier this week, investigators said they believed that the bombs were made of a high-grade, military-style explosive. The officials, speaking in return for anonymity because the investigation is continuing, said the material found in the home was TATP, a highly volatile explosive. While TATP is made