The explosives found on a high-speed rail track on Friday were of the same type and brand used in the Madrid train blasts, Spain has confirmed.
Spain says it is too early to say who planted the bomb
But Spanish Interior Minister Angel Acebes said it was still too soon to draw any conclusions about who planted the unexploded device.
Army vehicles and helicopters are patrolling key parts of the Spanish rail network following Friday's alert.
The line between Madrid and Seville has re-opened after a search of the tracks.
Mr Acebes said the bomb found on Friday contained an explosive called Goma 2 Eco - the same Spanish-made dynamite used in the 11 March bombings which killed 191 people and injured hundreds.
Mr Acebes, part of the outgoing Spanish Government, said it was not yet certain whether the detonator attached to the unexploded bomb was the same as those used in the Madrid blast.
BOMB AT MOCEJON
Bag contained 10-12kg of explosive
Bomb contained Goma 2 Eco, used in Madrid attacks
Bomb had no 'trigger'
But in both cases, the detonators appeared to be devices commonly found in the mining industry, he told reporters.
The unexploded bomb was found on the train line at Mocejon, in the Toledo area, about 60km (40 miles) south of Madrid.
Bomb disposal experts found between 10 and 12kg (22-24lb) of explosives, which were connected to a detonator with a 135m cable.
Mr Acebes said the bomb had failed to explode because it had no trigger - suggesting that those responsible may have been scared off by security guards as they were planting the bomb.
On Friday, a Spanish judge brought provisional charges against a 15th suspect in the Madrid bombings.
Otman El Gnaout, a Moroccan, is accused of collaboration with or membership of a terrorist organisation.
The judge also released four men on Friday, two of them Syrians, one a Moroccan and one a Spaniard.
The BBC's Katya Adler, in Madrid, says despite a call by Mr Acebes for people to avoid rushing to conclusions, much of the Spanish press and the general public have decided that Islamic extremists planted this latest bomb.
She says Spaniards have reacted in stunned disbelief at the news of another attempted attack.
Our correspondent adds that trains and motorways are packed across the country at this time of year as millions of families set off on their Easter holidays.
Several newspapers reported on Saturday that the Spanish embassy in Egypt had recently received a letter signed by the Abu Hafs al-Masri Brigades threatening to attack Spanish embassies and Spanish interests in north Africa and the southern and eastern Mediterranean region.
The letter warned that the attacks would go ahead unless Spain withdrew its troops from Iraq and Afghanistan within four weeks, El Mundo reported.
On Friday, Nato foreign ministers agreed a series of measures to fight terrorism, including more sharing of intelligence.
And in Washington, security officials warned that the Madrid bombings had increased the level of concern about a possible attack on US soil.
The FBI and the department of homeland security said extremist groups might try to bomb buses and rail lines in the US, hiding explosives in luggage and carry-on bags.