Kenyan investigators have identified one of the extremists involved in the attack on a hotel and shopping complex in Nairobi as the son of a Kenyan military officer. There were conflicting reports on Friday about whether the extremist was among the five attackers who died in the assault, or one of 11 people who have been detained by police since the Tuesday attack.

Kenyan authorities have said 21 people, including one police officer, were killed by the attackers. The al-Shabab extremist group, which is linked to al Qaeda and based in neighboring Somalia, claimed responsibility.

Intelligence officials in the east African nation confirmed to CBS News on Friday that the al-Shabab cell had been scouting the upmarket dusitD2 hotel complex since at least December 2016. CBS News has obtained security camera video from that month which shows three of the attackers walking through what looks like a parking garage at the complex.

An image of video from a security camera at the dusitD2 hotel and business complex in Nairobi, Kenya, taken on Dec. 28, 2016, shows three of the al-Shabab extremists who would return to attack the complex in January 2019, walking through a parking garage.

CBS News also obtained on Friday a leaked letter sent by Kenya's intelligence community to the national police, warning officers in the capital to be alert as al-Shabab was known to be plotting an attack in Nairobi. The October letter warns that a cell of six al-Shabab militants was believed to be "planning to attack" two sites, including the treasury, in the capital between Oct. 28 and Nov. 4.

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The two buildings mentioned in the intelligence brief are about three miles from the Dusit Hotel complex.

Five of the suspects arrested in connection with the attack appeared in court on Friday, and a judge said police could continue to hold them for 30 days to allow investigators to complete their work. Two of the five suspects were arrested on Wednesday, soon after the 20-hour siege came to an end.

Director of Criminal Investigations George Kinoti said on Wednesday the suspects were arrested in Nairobi. He said they were being treated as key suspects because they were in contact with the gunmen killed at the dusitD2 complex.

Investigators said they found bomb-making material at the house of one of the suspects.

Son of an army officer

The military officer, who is not believed to have been involved in the attack, was summoned for questioning about when he last saw his son and any other details, a senior police official said. The official spoke on condition of anonymity because he was not authorized to speak to the media.

The official said the suspect earlier was thought to be dead because his car was found at the dusitD2 hotel that was attacked on Tuesday. However, authorities now believe he may be among those detained, the official said.

"We are seeking further to know if he was at the scene and escaped, or he did not go there at all," the official said.

The Daily Nation, a Kenyan newspaper, said the alleged extremist's father is a sergeant with the Kenya Defence Forces. His mother was detained in Isiolo town and taken to Nairobi for questioning, the newspaper said.

Kenya's government announced on Friday that security guards manning public places would be licensed to carry fire arms on duty after a new training and vetting process.

Jason Spindler remembered by friends and family

One American, Jason Spindler, was killed in the terror attack. His parents were due to arrive Friday in Kenya to collect their son's body. CBS News correspondent Debora Patta reported that the couple was also planning to spend time in the country celebrating his life with the diverse community of friends Spindler had built around him.

A former investment banker who survived 9/11 because he was late for work that day, Spindler later gave up a high-powered Wall Street career for the chance to make a difference. He was, his parents say, one of the good ones.

"Jason, once you met him, you knew this guy was going to change the world. He had a way about him that everyone would like him," his father, Joseph, told CBS News.

His passion for change led him to Kenya where, with co-founder Patricia Chin-Sweeney, he started a company that aims to reduce poverty through investing in developing economies.

"He believed you could invest, you could build businesses and they would be a means of stabilizing economies and reducing the need for terrorism," Chin-Sweeney said.

The company's offices are housed in the complex that was stormed by Al-Shabab gunmen on Tuesday afternoon. Jason was having a late lunch there when a suicide bomber blew himself up just a few meters away.