Read more news from CNN

Nairobi attack shows attempts to neutralize terror threat have failed

Nairobi, Kenya(CNN) The terrorists used ammunition with such merciless abandon their automatic gunfire tore through the Nairobi dawn like a chainsaw in long screaming bursts. Grenade after grenade, detonations thumped among the buildings.

How could a gang of five -- one suicide bomber and his four gunman accomplices -- get into the heart of the Kenyan capital so heavily armed, and with what seemed like unlimited ammunition?

The answer lies along Kenya's border with neighboring Somalia -- the home of Al-Shabaab terrorists who claimed to have carried out the attack on the Dusit II complex in Nairobi's Westlands.

A woman hides behind a car at the scene of the attack in Nairobi's Westlands suburb on January 15.

The al Qaeda-linked group has been able to exploit a largely unguarded border that stretches for hundreds of miles separating the most remote reaches of Kenya from the most lawless nation on the planet.

It also lies in Kenyan corruption. A UN report, published at the end of last year, details how Kenyan border security agents had been easily bribed by known al-Shabaab terrorists not to search their cars.

The UN investigation, published in November, by the Somalia and Eritrea Monitoring Group, detailed how plans to attack targets in Nairobi with bombs and small arms were thwarted early last year. And that the weapons and the terror team behind it had traveled through Kenyan border checks having paid bribes of just US$20.

The Kenyan government has long denied allegations of corruption -- and rejected the findings of the UN report.

Al-Shabaab have used their bases in Somalia to launch attacks before. Notably the Westgate shopping mall massacre, also in Westlands, in 2013.

Its main effort has been fighting African peacekeepers and staying out of the sights of American drones and bombers, which have been attempting to pick off the group's senior management with airstrikes for several years.

Group shrugs off US strikes

But Al-Shabaab has hit back. Over the Christmas and New Year period the extremists were able to drop six mortar rounds into the accommodation blocks at the UN headquarters in Mogadishu.

The UN HQ sits inside the heavily guarded Mogadishu airport compound, a mini-city of containers and bunkers that also serves as the HQ of the African peacekeeping operation, Amisom.

The African forces have been slowly scaling back their operations while Al-Shabaab has relentlessly targeted the capital with bombing attacks no matter how frequently they have been hit from the air by American technology.

Residents survey remains after bombings in Mogadishu.

The extremist group generates vast amounts of funds from charcoal exports and by gouging aid agencies and ordinary Somalis in protection rackets and road tolls.

And with a Somali population inside Kenya, many of them refugees, of at least 1.5 million, the terrorists are able to move around the country almost invisibly.

A messy civil war has raged in Somalia since 1991, creating generations of trained killers and arms smuggling networks that extend across Africa and the Middle East, thanks to a porous border and corrupt officialdom. It is surprising then that Kenya has not seen more attacks.

As the UN report ominously noted after the foiled plot of February last year: "Several known members of the plot escaped arrest, and the attacking team -- which likely comprised five individuals, on the basis of the number of captured rifles -- may still be at large in Kenya."

We now know that five terrorists took part in the Dusit rampage. They may, or may not, have been a different gang -- but their aims and methods were the same.

Outbrain