The device was found in the driver's seat, National Police Commissioner Dan Eliasson said, but it was not known whether it was a bomb.
Mr Eliasson also said the suspect in custody was from Uzbekistan, 39, and known to security services.
The hijacked lorry driven into Ahlens department store in central Stockholm.
Four people were killed - 10 remain in hospital, including a child. Two are in intensive care.
The man, previously known to Swedish intelligence services as a marginal figure with no clear links to extremist groups, is suspected of mowing down pedestrians on a busy shopping street and smashing through a store front on Friday.
Police said they were increasingly certain they had arrested the man who carried out yesterday's truck attack in the capital Stockholm.
"Nothing indicates that we have the wrong person, on the contrary, suspicions have strengthened as the investigation has progressed," Dan Eliasson, head of Sweden's national police, told a news conference on Saturday.
The man, detained on Friday night on terrorism charges after the attack in the heart of the capital, appeared to have acted alone but "we still cannot rule out that more people are involved," he said.
Court-appointed lawyer Johan Eriksson said he had met with the suspect on Saturday but declined to give further details about his client.
Police did not name the detainee, but said he was from the central Asian republic of Uzbekistan and that he had seemed peripheral in intelligence reports.
"We received intelligence last year, but we did not see any links to extremist circles," Sapo security police chief Anders Thornberg said.
Eliasson said there were "clear similarities" to an attack last month in London in which six people died, including the assailant who drove a hired car into pedestrians on a bridge.
The head of Swedish security police, Anders Thornberg, said the suspect had not been "part of any of the security police's ongoing investigations", but was "a person who has previously figured in our intelligence flow".
Local authorities in Stockholm, where flags flew at half mast, said 10 people including a child were still being treated in hospital, with two adults in intensive care.
Sweden was set to hold a minute's silence at midday on Monday to mourn the dead. Police said they were maintaining a heightened presence, fearing copycat attacks.
The mayor of Stockholm, Karin Wanngard, reiterated that Stockholm was open.
"This is not an attack that's about the colour of your skin," she said.
"We can show that with good integration, with an openness and a friendly behaviour, we are stronger together and it doesn't matter where you come from."
Sweden has generally low crime rates, and has been ranked as one of the safest countries in the world.
In 2010, two bombs detonated in central Stockholm, killing the attacker - an Iraq-born Swedish man - and injuring two others.
In October 2015, a masked man who was believed to have far-right sympathies killed a teacher and pupil in a sword attack.
In February, US President Donald Trump cited a non-existent terror attack in Sweden, and blamed it on the country's asylum policy - baffling many Swedes.
Sweden has taken in nearly 200,000 refugees and migrants in recent years - more per capita than any other European country.
However, there was a drop in numbers last year after the country introduced new border checks.
Separately, Sweden is believed to have the highest number of Islamic State group fighters per capita in Europe.
About 140 of the 300 who went to Syria and Iraq have since returned, leaving the authorities to grapple with how best to reintegrate them into society.
- Reuters / BBC