Egyptian officials suspect a violent branch of the Muslim Brotherhood, called the Hasm Movement, is responsible for a Dec. 28 terrorist attack that killed three Vietnamese tourists and an Egyptian tour guide.

Authorities believe that the attack on a tourist bus near the Giza pyramids was in retaliation for a preemptive strike Egyptian security forces carried out eight days earlier that killed eight Hasm militants and saw four others arrested.

The raid on the Hasm hideouts aimed to thwart planned Christmas and New Year's attacks. Officials found stashed assault weapons, remote-controlled planes, a drone chassis and materials used to make improvised explosive devices (IEDs) that have been used in attacks similar to the tourist bus bombing.

"There were two cars belonging to Hasm seized with similar explosives used in the operation," Egyptian High Council for Counter-terrorism and Extremism General Khaled Okasha told UAE's Al-Ghad television. "There were alerts that these Hasm cells desire to exploit this time of the year to conduct operations."

The U.S. State Department designated Hasm (Arms of Egypt Movement) and its sister group Liwa Al Thawara as terrorist organizations a year ago. Both groups are associated with the Muslim Brotherhood. Hasm issued an emotional obituary when former Brotherhood Supreme Guide Mehdi Akef died in 2017, describing him as "the champion mujahid who fought the British Occupation, the tyrants Nasser, Sadat and Mubarak and the fascist military regime that didn't care for his age and sickness."

In a televised confession after last month's raids, detained Hasm member Amr Ayman Alyeddin admitted being an active member of the Muslim Brotherhood. He said that he joined the Brotherhood's military cells through a recruiter named Yasser Al Zeiny, who is in Egyptian police custody. Alyeddin says Al Zeiny connected him to a trainer who took him to terrorist camp outside Cairo. Alyeddin was taught how to use weapons and given religious law training.

Another cell member, Saladin Hamed, said he was told that the Brotherhood leadership abroad has reformed the cells into military units with codenames. The cells operate independently.

Egyptian police raided two terrorist hideouts in Giza in response to the bus attack, reportedly killing 40 terrorists.

Hasm is the Muslim Brotherhood's most effective militant wing, Okasha said, as Liwa Al Thawra has been weakened by the government. Hasm was formed in 2014, while Liwa Al Thawra emerged in 2016 as a splinter group from Hasm created by Brotherhood leader Mohamed Kamal. Kamal was killed in October 2016 in an exchange of gunfire with security officers.

Meanwhile, authorities also are investigating why the tour bus carrying the Vietnamese tourists strayed from a heavily patrolled route.

While the attack on the Vietnamese tourist bus created fears that Egypt's recovering tourism industry would suffer, there are not yet any indications of any declines in visitors. Egyptian officials point to a sharp drop in terrorist attacks in the country. A new report says there were just eight terrorist attacks last year, down from 222 in 2014.

Aside from the threats posed by ISIS-affiliated groups in North Sinai, Hasm remains a challenge for Egyptian security forces as it plots to disrupt the improved security that the country is seeing. Other Hasm operations in the past few years include failed assassination attempts on former Grand Mufti Ali Gomaa, Deputy Attorney General Zakaria Abdulaziz and Judge Ahmed Abou El Fotouh – who presided over the prosecution of ousted Egyptian President Mohammed Morsi of the Muslim Brotherhood.

Hasm also claimed responsibility for attacks on police, including the 2017 assassination of national security officer Ibrahim El Azazi.

Its attacks are not limited to Egyptian targets. Hasm claimed responsibility for a small explosion outside the Myanmar Embassy in Cairo in October 2017 to avenge Rohingya Muslims. "This bombing serves as a warning to the embassy of murderers, killers of women and children in the Muslim Rakhine State, and in solidarity with the sons of this weakened Muslim population," a Hasm statement said.

Hasm protested after the United States' terrorist designation was issued. After the U.S. Embassy in Cairo issued a travel warning last May advising of a possible Hasm attack, the group played innocent. "We are the #Resistance and we are not terrorist," a Hasm statement said. "There is no need to worry. We are #Muslims, not killers."

The IED attack on the Vietnamese tourist bus indicates there is plenty of reason to worry.

Hany Ghoraba is an Egyptian writer, political and counter-terrorism analyst at Al Ahram Weekly, author of Egypt's Arab Spring: The Long and Winding Road to Democracy and a regular contributor to the BBC.