The injured were taken to Malindi Sub County Hospital and on Wednesday morning, sources told the Nation that two were in critical conditions.
Mr Boinnet identified the volunteer as an Italian woman aged 23.
Police say five people, among them a 12-year-old boy were shot and seriously injured during the attack that occurred at a shopping centre in Chakama area, about 80 kilometers West of Malindi Town.
Neither the reasons for the attack nor the identity of the attackers has been established.
"We assure the public that we are sparing no effort in tracking down the criminals and rescue the victim", the National Police Service said.
"The European lady got out of her room, instead of lying on the ground, to enquire what was going on", an eye-witness told local media.
She was taken after dark on Tuesday evening, with the attackers firing "indiscriminately at residents" with AK-47 assault rifles during the raid.
Sylvia is reported to have been abducted by gunmen armed with AK47s, who begun shooting without discrimination at Chakama Trading Centre.
The armed group was also responsible for the 2013 attack on a shopping mall in the capital, Nairobi, in which almost 70 people were killed.
Somali-based Islamic militants have been blamed for a spate of kidnappings of foreigners at Kenya's coast.
In 2011, Kenya sent troops into the neighbouring country to fight the al-Shabab group in response to the abduction of four foreign nationals, one of whom was a cancer-stricken French woman who died in captivity.