Four of the accused men are in government custody and appeared before a military tribunal in Mogadishu on Monday, while the fifth defendant is on the run.
Among those charged is Hassan Aden Isak, who is accused of driving a vehicle that allegedly was intended to be used in a second suicide bombing that day.
The chief of the military tribunal, Col. Hassan Ali Nur Shute, also accused Isak of coordinating the attack and working as head of explosions and assassinations in the Mogadishu area for the militant group al-Shabab.
Somali authorities have said they have no doubt al-Shabab was behind the October attack, although the group did not claim responsibility. Al-Shabab has carried out dozens of deadly suicide bombings by car and on foot in Mogadishu over the past decade, often targeting hotels, restaurants and other public places where casualties are bound to be high.
Isak was arrested on the day of the attack by security forces after trying to walk away from a Toyota Noah minivan. The minister of internal security, Mohamed Abukar Islow, said Isak did not intend to detonate the minivan himself but was waiting for another driver to take over from him.
According to Somali intelligence sources, al-Shabab intended the big truck to pass through road checkpoints, link up with the minivan and launch a complex attack on the recently opened Turkish military training base in Mogadishu.
Also charged Monday was Abdullahi Ibrahim Hassan Absuge, who is listed as the owner of the truck that exploded. He was charged in absentia for terrorism.
According to the case against him, Absuge purchased the truck on Aug. 18. The following month on Sept.13, the truck started making trips between Mogadishu and the Lower Shabelle region, bringing farm produce and other goods into the capital.
The man behind the wheel at the time was Hussein Aden Madey, the suicide bomber who eventually detonated the explosives at K-5 junction.
Intelligence officials say the trips were a "dry run" for the attack and an attempt to familiarize the driver with government forces manning security checkpoints along the road linking Mogadishu to Lower Shabelle farmlands where al-Shabab eventually loaded the truck with explosives.
According to a security source, Absuge was also the owner of a second truck that was detonated at a vegetable market in Mogadishu's Waberi district on Nov. 26, 2016. That explosion killed 20 people.
Absuge's whereabouts are not known, and the government is offering an unspecified reward for information leading to his capture.
Two others — Abdiweli Ahmed Diriye, 32, and Mukhtar Mohamud Hassan, 43 — also appeared before the military court on Monday. They are accused of talking to the security forces at the Siinka Dheer checkpoint to "bail out" the driver and helping him pass through.
The fifth defendant, Abdul Abdi Warsame, 35, is accused of paying a tax at a second checkpoint where upon receipt, security forces let the truck pass through.
Only at the third checkpoint near the old U.S. embassy in Mogadishu did Madey, the driver, find it difficult to pass. When soldiers at that checkpoint became suspicious,Madey sped away. Moments later, he detonated hundreds of kilograms of explosives at the busy K-5 junction.
Shute said on Monday that a total of 776 people were either killed or maimed in the explosion.