By Sandi Sidhu and Pradas Subramaniam, CNN

(CNN) -- The sister of the suspected ringleader of Sri Lanka's deadly Easter Sunday bombings has told CNN up to 18 of her family members are missing and feared dead since the attacks and subsequent raids.

More than 250 people were killed and at least 500 injured in a series of coordinated suicide bombings at churches and hotels across the island April 21.

A week on, the country is still on high alert with warnings there could be more attacks in the coming days, MPs told CNN Sunday.

Mohamed Hashim Mathaniya is the sister of Mohamed Zahran Hashim, the man Sri Lankan authorities believe was one of the leaders of the attacks.

He appeared in a video released by an ISIS-linked news agency before blowing himself up on Easter Sunday.

Speaking to CNN on Saturday, Mathaniya said she identified her brother from photographs of his body parts at the police station earlier in the week.

"Five men went missing after the attacks (on Sunday). They were my three brothers, my father, and my sister's husband," she said.

Suspected militants raided on Friday

On Friday night, 10 civilians -- including six children -- were killed along with six suspected terrorists after a shootout between police and alleged militants in the town of Sainthamaruthu on Sri Lanka's eastern coast.

At daybreak Saturday, a gruesome scene was revealed at the raided house -- charred bodies and a roof blown off during three explosions.

One of the militants killed in that raid has been identified as Mohamed Niyas, a prominent member of the local extremist group National Tawheed Jamath and Mathaniya's brother-in-law.

"It did not hit me until I saw the bodies of the men and women. When they said six children, I thought whether they could be the people related to me," Mathaniya told CNN.

"Among the women, there were five women there in the house. The wives of my three brothers, my younger sister, and my mother. There were altogether seven children."

Witnesses told CNN one explosion during the raid turned the Sainthamaruthu house "into fire."

Mathaniya said her brother Zahran Hashim's wife and daughter are currently in the hospital. Police confirmed that after Friday's house raid, a woman and child with life-threatening injuries were taken to hospital.

Authorities continue investigations

One wounded suspect fled Friday's shootout on a motorbike, and another suspected terrorist could be on the run as well, Sri Lanka's military said.

Earlier Friday, authorities had seized a large cache of explosives, 100,000 ball bearings and ISIS uniforms and flags from a garage a few miles from the raided property.

ISIS has claimed responsibility for the Easter Sunday attacks, but a link between the attackers and the terror group has not been proven. Authorities blame National Tawheed Jamath, which has not claimed the attacks.

Warnings of more attacks

The raids are part of a nationwide hunt for the perpetrators of the Easter Sunday attacks. In the last 24 hours, at least 48 suspects have been arrested, National Police Spokesman Ruwan Gunasekera said Sunday.

Meanwhile three MPs told CNN that their personal security officers have received an internal memo from the VIP Security Division of the Sri Lankan police, warning that there could be more attacks in the coming days.

The memo said the possible attacks were planned by the "same perpetrators" as those thought to be behind the Easter Sunday bombings.

Sister of bomber identifies 3 family members in video

Mohamed Hashim Mathaniya also identified her father and two brothers in a video purportedly taken minutes before Friday's shootout.

The video, widely circulated on Sri Lankan social media, shows three men saying in Tamil that they will "teach a lesson" to those who "are destroying Muslims who have come to this part of the country."

The men urge people to leave their jobs to take up jihad. CNN has not been able to independently verify the authenticity of the video.

Mathiniya said the three men in the video were her father, Mohammed Hashim, and her brothers, Mohamed Hashim Rilwan and Mohamed Hashim Zainy.

India warned preacher was planning attacks

Weeks before the Easter Sunday bombings, India's intelligence service warned its Sri Lankan counterpart that Zahran was planning an attack on churches and hotels.

The radical Islamist preacher was known to the authorities and local Muslim community for years as a dangerous and violent figure.

In videos Zahran posted online, he preached hate and violence and called for attacks on other Muslims, Buddhists and Christians.

Both Christianity and Islam are minority religions in Sri Lanka, with each accounting for under 10% of the population. The vast majority of Sri Lankans identify as Buddhist.

In his hometown of Kattankudy -- about an hour's drive north from Sainthamaruthu -- locals told CNN they were terrified of Zahran, even after police had confirmed his death in the attacks.

They painted a portrait of a community that was growing increasingly radical, in part due to an influx of foreign money for mosques and schools, as moderate Muslims were the subject of harassment and even violence from and supporters of the preacher.