SAN DIEGO -- The effort to rout out every San Diego connection to the terrorist attacks continued Thursday, with investigators speaking again to an Islamic English professor at his Lemon Grove home.

The professor is only considered to be a material witness to the case, but FBI officials are interested to learn what he knows about the two men who rented a room in his home.

Neighbors of Dr. Abdusattar Sheikh are used to seeing people come and go from his home at all hours. It used to be boarders, now it's FBI agents.

Sheikh has seen a lot of the FBI since the attacks. Agents are eager to learn what he may know about a number of people associated with the attacks. The investigation, as it extends from Sheikh, breaks down into the following categories: suspects, a witness and an acquaintance.

Suspects

Suspects Khalid Al-Midhar and Nawaf Alhamzi. Midhar and Alhamzi are believed to have been among the five men who hijacked Flight 77 -- the flight that slammed into the Pentagon. Both men rented a room from Sheikh.

When Sheikh learned that two former boarders were accused of committing the attacks, he told reporters that he was shocked, and welcomed FBI agents in to search his home.

Meanwhile, the Associated Press is reporting that at least five of 19 suspected hijackers in the attacks stayed in Las Vegas in recent months.

Mohamed Atta, Marwan Al-Shehhi, Hani Hanjour, Nawaf Alhazmi and Ziad Samir Jarrah visited Las Vegas between May and August. The FBI is asking for anyone with information about the men's visits to call.

Witness

FBI agents are also talking to 24-year-old Irvine, Calif., college student Ramez Noaman, about his knowledge of Alhamzi and Midhar.

Mail in Ramez' name was reportedly sent to Sheikh's house.

"The FBI told me that my cousin knows some people from San Diego who were involved in the hijackings," Noaman's cousin, Akram Mokred, 23, of Yemen, said. "I knew Ramez the longest. We grew up together. But I don't know who he knew in San Diego."

Acquaintance

On Thursday, agents were back at the Sheikh home to ask about Mohammed Atta. Atta is believed by FBI agents to have been the Sept. 11 operation's ringleader.

A Lemon Grove resident has told 10News that she had once seen Atta in the neighborhood. The resident, who asked that her name be withheld, told 10News that Atta had been hanging out at the house of one of Sheikh's acquaintances.

Meanwhile, the three San Diego men -- Mohbar Abdallah, Osama "Sam" Abdallah and Yazeed Al-Salmi -- being held by the FBI as material witnesses were sent to New York, according to 10News.

The men are believed to have been acquaintances of Alhamzi and Midhar. The families of the three men insist that they are innocent of any crime.