About 19 terrorists trained by al-Qaeda were involved in all the four terror attacks. The four pilots, who crashed flights into the WTC building, had trained at US flying schools. The pilots of AA11 flight, Wail al-Shehri and Waleed al-Shehri, were brothers. So was the case with the pilots of A77 flight, Navaf al-Hasmi and Salim al-Hasmi. Box cutters and knives were used for hijack. Among the terrorists, the eldest member was 33-year-old at the time of the attack. Others were in the 20 to 29 age group. The 19 terrorists had entered the United States between January and July 2000.