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Ben Curtis, File, Associated Press
In this Tuesday, Nov. 25, 2014 file photo, a protester lays red-painted crosses on the street, symbolizing the blood of the dozens of non-Muslims who were singled out and killed in the recent attack on a bus in Mandera by Somali militant group al-Shabab, outside government offices in downtown Nairobi, Kenya. Kenya's police force said Wednesday, Feb. 25, 2015 that 173 people were killed in 2014 in attacks by Islamic extremists, the highest number in the three years that Kenya has experienced violence blamed on neighboring Somalia's al-Shabab militants.

NAIROBI, Kenya — Islamic extremists killed 173 people in Kenya in 2014, the highest number in the three years that Kenya has experienced violence blamed on neighboring Somalia's al-Shabab militants, Kenya's police force said Wednesday.

The 2014 death toll is even higher than that of 2013 when extremists killed 67 people in the assault on Nairobi's Westgate Mall.

Kenyan police said 312 people were killed in the country in militant attacks since 2012 and 779 people were wounded during the period.

Nairobi, Mombasa, Garissa, Mandera and Wajir were towns where the attacks took place between 2012 and 2014. The targets were security officials, places frequented by tourists, passenger service vehicles, churches, malls and other crowded places, said acting police chief Samuel Arachi.

Al- Shabab has vowed retribution for Kenya sending troops to Somalia. Kenya deployed its troops to Somalia in October 2011 to fight the extremist rebels who had carried out cross-border attacks including the kidnapping of four Europeans. Kenyan troops are now part of a multinational African Union force bolstering Somalia's weak government against an insurgency by al-Shabab which is linked to al-Qaida.

Al-Shabab, in a video released Saturday, threatened more attacks on Kenya.

Al-Shabab gunmen shot dead at least 60 non-Muslims in two separate incidents in northern Kenya in late November and early December.

A Kenyan official said Tuesday the government is looking at new options to control the threat including constructing a wall or gullies on parts of the 682 kilometer (423 mile) border with Somalia.

The official who insisted on anonymity because he is not authorized to speak to the press said there are discussions about how to create a buffer zone with Somalia.

Systematic corruption prevents Kenya from effectively tackling extremist attacks, said an analyst.

"The systemic corruption that afflicts all our institutions infects our security services, too. This continues to be the dead weight our efforts against terrorism have to carry," said Kenyan corruption expert John Githongo, by email from California where he is a visiting scholar at Stanford University.