years said the group, based in the southern part of Pakistan's richest and most populous province Punjab, had sent hundreds of fighters to Syria.
"The new cadre of militants going to Syria and Iraq includes mostly educated people with middle-class backgrounds," the intelligence officer told AFP.
Over the past decade, the patchwork of militant groups that make up the Pakistani Taliban have largely focused on waging a domestic campaign against the government and armed forces. But a former LeJ militant who produces online propaganda material for terror groups said for young militants in Pakistan, all the talk now is of IS and the Middle East.
"Many jihadists, particularly from Punjab, went to fight in Syria and some were martyred," he said. "Unlike the past, news from Syria, Iraq and Yemen is the most debated and shared item on jihadist forums in Pakistan."
Security analyst Amir Rana said LeJ had had fighters in Iraq since 2013, and even set up a training camp there.
"The ideological and operational association between Pakistani militant groups and IS is not new, Pakistani militants were part of IS since its inception," he told AFP.
"The actual threat for Pakistan is the return of LeJ militants fighting in Iraq and Syria, as they would add to the sectarian violence here."
Silent surge
LeJ, founded in 1996, was behind some of the worst attacks on Shiites in Pakistan's history, including two huge bombings in Quetta in 2013 that together killed nearly 200 people.
The security official said the group was now seeking to expand its operations.
"LeJ is growing from an anti-Shiite organisation to an organisation with trans-national interests," he said.
The group has been accused of carrying out attacks in Afghanistan and has also begun targeting Christians, Hindus and other Muslim minorities.
"For the past two years, there is evidence that the organisation is involved in attacking minorities in urban centres where they have established strong bases, especially in Karachi," he said.
"But LeJ has claimed responsibility for hardly any of those incidents — usually militant organisations with no structural or organisational existence have claimed responsibility for attacks carried out by LeJ," he added. He said LeJ maintained a strict cellular structure, with individuals in one unit unaware of the existence of others, and sometimes drew militants from other groups for specific missions.
Educated and radical
On May 20, the Sindh chief minister, Qaim Ali Shah, announced the first arrests in connection with the Karachi bus attack.
He said four "highly educated" suspects had confessed their involvement, including a graduate of the city's prestigious Institute of Business Administration (IBA).
The detentions of the alleged plotters, who police believe coordinated the gunmen on the ground, caused
some surprise in Pakistan, where militancy has been regarded as the preserve of the poor.
But the arrests come as no surprise for officials investigating the case. The intelligence official overseeing the Karachi investigation said the abundance of jihadist literature on the web was attracting educated people from the middle class.
Another senior intelligence official said people with strong academic backgrounds were being increasingly radicalised following the Syrian conflict.
"These ´educated jihadists´ are embedded in society, they have normal lives and keep their ideologies alive through the Internet — that's why it is difficult to place tabs on them," he said.