The investigation into Tuesday's head-on train crash in southern Italy that killed 23 people is focusing on the antiquated alert system on the line.
They are looking into one of the black boxes recovered at the scene of the collision, on a remote single-track line north of the city of Bari.
The system relied on telephone calls and "human error" remains the main line of inquiry, reports say.
Dozens of rescue workers are still searching the wreckage.
The inquiry is focusing on the lack of automatic signalling system on a small part of the Italian railway network.
The stretch of track between the towns of Andria and Corato in the southern region of Apulia where the crash happened did not have an automatic alert or brake system.
It relies on station masters phoning one another to advise of trains running on the single track.
"Surely one of the two trains shouldn't have been there," railway police Cdr Giancarlo Conticchio is quoted as saying by the Associated Press news agency.
"And surely there was an error. We need to determine the cause of the error," Mr Conticchio said.
The collision took place in good weather at 11:30 local time (09:30 GMT) as the trains were travelling at high speed between the towns of Bari and Barletta.
Both trains had four carriages.
Local officials said it was difficult to say how many people were on board the train at the time as there was no list.
Fifteen people are being treated in hospital for their injuries, and some of them remain in a critical condition. Officials initially put the number of dead at 27 but local authorities said later that 23 people had been killed.
Most passengers had no warning of the impending disaster which one witness compared to a plane crash.
One woman, eight months pregnant, described the moment of impact,.
"I was thrown forward, I don't know what happened, it all happened so quickly. I saw my mother on the ground, my father and my sister bleeding. The people on the train helped us."
One elderly man said he was knocked to the ground, while his wife described how she came across body parts as she freed him.
"I pulled him from under the debris, myself barefoot, from under the debris and metal," she told local TV. "I went to my husband screaming. I pulled him by the legs and feet. I climbed past people in pieces, how sad. There was nothing I could do."'
Cranes and heavy lifting equipment have been clearing the wreckage. The army is helping the operation.
Relatives of the victims are going to the mortuary in Bari to help to identify the dead.
Italian PM Matteo Renzi visited the site on Tuesday and promised a full investigation. He described the death toll as unacceptable.
Italy's deadly train accidents
November 2012: Six people believed to be Romanian farm workers are killed after a van they were travelling in is struck by a train as it crosses railway tracks in Calabria
June 2009: Freight train carrying liquefied petroleum gas derails in Viareggio, causing a large explosion. More than 30 people die
January 2005: A head-on collision between a passenger and a freight train near Crevalcore kills 17
July 2002: A passenger train derails in Rometta Messina, killing eight people
April 1978: Two trains collide near a ravine next to Murazze Vado. Some of the carriages fall into the gorge, killing 42
Sources: Ansa, Corriere della Sera