Libyan police found Mohammed Yousri Yassin, brother of the man who died in a bomb blast on 30 April, hiding in a Libyan home rented by an Egyptian working as a hotel cook, security officials said on Sunday on customary condition of anonymity.
Yassin, 17, was extradited to Egypt on Saturday and was also being sought in relation with a 7 April suicide bombing in a crowded Cairo bazaar that killed three tourists.
The officials said Egypt's intelligence service informed Libyan authorities of Yassin's whereabouts in Libya after learning he had crossed the border shortly after the attack.
"It was a well coordinated effort and the Libyans were very cooperative," an official said.
Yassin's brother blew himself up,
injuring several people
Neighbouring Egypt and Libya are bound by a bilateral and a joint pan-Arab treaty of security cooperation that permits extradition of terror suspects.
The detainee is the younger brother of Ehab Yousri Yassin, who police claimed jumped from a Cairo bridge near the Egyptian Museum on 30 April to evade a police chase before blowing himself up with a nail-packed bomb.
The blast was followed by a shooting in Cairo less than two hours later, reportedly involving the bomber's sister and fiancee, whom police claimed later killed themselves.
The two attacks wounded nine people, including four foreigners.
After the attacks, police started an intensive search for the younger Yassin, who is suspected of being involved. Police detained about 200 people from an area north of Cairo where the suspects once lived.
Two militant groups have claimed responsibility for the attacks - the Mujahidin of Egypt and the al-Qaida-influenced Abd Allah Azzam Brigades. Neither claim could be verified.