The man charged with carrying out the deadly truck attack in Stockholm in April 2017 was in contact with alleged Islamic State (IS) militants from Tajikistan, a joint investigation by RFE/RL and a Swedish news agency has found.

The investigation by RFE/RL's Uzbek and Tajik services and Swedish news agency TT found that Stockholm terror suspect Rakhmat Akilov was in direct contact with alleged senior members of the IS group from Tajikistan before, during, and after the deadly April 7 attack.

Akilov, a 39-year-old Uzbek asylum seeker, is suspected of stealing a truck and plowing into a crowd of people on a pedestrian street in Stockholm, killing five.

Tajik Interior Minister Ramazon Rahimzoda told RFE/RL that Swedish authorities had not been in contact with Dushanbe over Akilov's ties to suspected Tajik IS members.

IS did not claim responsibility for the Stockholm attack.

Akilov is in pretrial detention in Sweden and his trial is expected to start on February 13. Akilov faces a life sentence if convicted.

Akilov, a construction worker, was arrested a few hours after the attack and police said he confessed the next day.

According to the investigation, Swedish police found the contacts of several people described as "terrorist-related" on messaging apps like Zello and Telegram on Akilov's mobile phone.

The contacts had aliases like Abu Aisha and Abu Umar, which RFE/RL found to be the same names used by alleged Tajik IS recruiters.

Akilov is a member of the ethnic Tajik minority in Uzbekistan.

RFE/RL's Uzbek Service correspondent Sirojiddin Tolibov said they were aliases that only senior IS militants would use.

One contact of interest to investigators is Abu Osama Noraki. Akilov said during police questioning that he was speaking to Noraki, whom he called his leader, just before carrying out the truck attack. He said that Noraki "ruled my actions."

Tajik authorities have described a person with the same name as Noraki as among the IS group's "most dangerous recruiters in Tajikistan."

The Tajik Interior Ministry said it could not confirm if Noraki was in contact with Akilov, but said the former "participated in battles in Syria and Iraq" and was wanted by the authorities in Dushanbe.

WATCH: The failed Uzbek asylum seeker suspected of April's deadly truck attack in Stockholm led a double life and had several identities, according to a Swedish media report.

According to RFE/RL, another Akilov contact was Abu Umar, the same alias used by a high-profile Tajik IS commander.

Messages on Akilov's mobile phone purport to show the two discussing Akilov's "martyr operation."

Uzbek Foreign Minister Abdulaziz Kamilov claimed in April 2017 that Akilov had been recruited by the IS group after he left Uzbekistan in 2014 and settled in Sweden.

Akilov's lawyer, Johan Eriksson, said in January that his client "admits committing a terrorist act" and "admits that he should be convicted" and expelled from Sweden if he is not sentenced to life in prison.

Eriksson added that Akilov had not expected to survive the attack.

Prosecutors said after crashing the truck into the facade of a department store, Akilov set off an explosive device in the cab of the truck. He then fled the scene.

It is estimated that some 1,500 people from Uzbekistan have traveled to Iraq and Syria to join IS militants.

Several Uzbek citizens were also linked to major terrorist attacks abroad in 2017.

Sayfullo Saipov is the main suspect in a truck attack that killed eight people in New York City on October 31.

Abdulkadir Masharipov was identified by Turkish police as the man who carried out a gun-and-grenade attack at an Istanbul nightclub that killed 39 people on January 1, 2017. The IS group claimed responsibility for that attack.

With reporting by TT