NAIROBI - Kenya's anti-terrorism police have released four suspects arrested in connection with the recent grenade attacks at the main terminus in Nairobi, their lawyer said on Tuesday.

Lawyer Chacha Mwita said the four -- Sylvester Opiyo also known as Musa Osodo and three minors -- were released by police on Tuesday unconditionally and ordered to return to the anti- terrorism police offices on Friday for further questioning.

"My clients have been questioned for the third day and told to go home and return on Friday. We find it disturbing because they are innocent," Mwita told Xinhua by telephone in Nairobi.

The suspects were freed on Monday night before interrogated again on Tuesday by the Anti-Terrorism Police Unit (ATPU) to enable the authorities to unravel the cause of the Saturday's attacks, which have so far claimed nine lives and injured more than 60 others.

The three minors are students who were helping Opiyo offload his belongings from the vehicle since he was moving into a new house a few hours after the blast.

They were arrested on Sunday and detained until on Monday night when they were freed and later directed to report back for further interrogation early on Tuesday.

The authorities said Opiyo has been on the police radar since December 2011 when the police published his photograph along with his accomplish, Hussein Nderitu Abbas alias Mohamed, for being behind a spate of terror attacks in the east African nation.

The two surrendered to police on December 24, 2011 after police released their photos to the public terming them wanted persons. They were held and later released without charge.

Both individuals have cases pending in court, having been arrested in March 2011. The two suspects were set free following intervention from their lawyer.

The two surrendered after police released their photographs and details describing them as having links with Al-Shabaab and plotting terrorist activities.

Their lawyer Chacha Mwita said they opted to surrender to avoid any confrontation with the police.

There has been heightened security around the country with security agencies at an unprecedented state of alert amid the latest reports that Al-Shabaab have planned attacks against Kenya and foreign interests in the country.

The Kenya Defense Forces and Somalia's Transitional Federal Government troops have also intensified their operations in southern Somalia with military air strikes last week targeted at Al-Shabaab strongholds in southern Somalia.

Kenyan officials blame the Al-Shabaab insurgents or their sympathizers for the bombings and shootings, although armed bandits also operate in the border areas.