Suspected al Shabaab militants threw a hand grenade at a Kenyan police post on Monday in the northern town of Garissa close to the border with Somalia, police said, a day after two police officers were shot dead in the same town.Kenya has been rocked by a series of grenade and gun attacks since it sent troops into Somalia last October in pursuit of Islamist al Shabaab rebels whom it blamed for kidnapping its security personnel and Western tourists."A hand grenade was thrown, it dropped and exploded but nobody was injured," Garissa county commissioner Mohamed Maalim said, adding a gunfight ensued for about 10 minutes."The police fought back, secured the post and repulsed the gang," he said.Maalim said police suspected it was an al Shabaab attack, the third in two days, which follows the killing of two policemen in Garissa and a separate bombing in which three children attending a Sunday school were hurt when militants threw a hand grenade at a church in Nairobi.The attacks on Kenyan soil have intensified since Kenyan troops, alongside other African Union forces, launched an offensive against the southern Somali port of Kismayu, al Shabaab's last stronghold, forcing the rebels to flee."This is a heinous act by al Shabaab. They are out to cause chaos out of bitterness from their losses in Somalia," Maalim said. Security sources said police and paramilitary forces were searching for the gunmen.