Manhunt For French Gunmen, As Teenager Turns Self In

French Prime Minister Manuel Valls raised France's security to its highest level - "attack alert" - after the Charlie Hebdo bloodshed. That means there will be reinforced security at media company offices, major stores, religious centers and on public transport, Valls' office said in a statement.

All available forces have been mobilized, with civil and military reinforcements as part of this plan. In addition, regional authorities have been instructed to step up their vigilance.

U.S. counter-terrorism agencies are looking at a number of groups, including ISIS and al-Qaeda, that might be responsible for the attack. Charlie Hebdo has been singled out as a target for Islamist extremists in al-Qaeda's publication, Inspire.

Citing sources, the Agence France Presse (AFP) news agency reported that an 18-year-old suspect in the attack had surrendered to police. CNN has not independently confirmed whether the suspect has surrendered.

Police impounded a black Citroen in northeastern Paris similar to the one purportedly used by the attackers as a getaway car. Video from CNN affiliate BFMTV shows the vehicle being towed from Porte de Pantin, in Paris' 19th district.

Investigators will do a complete DNA work-up on the Citroen, including soil signatures that might suggest where the gunmen came from, a Western intelligence source briefed on the probe told CNN.

The same source said that French authorities are searching all travel records from the past 17 days to see whether any of the attackers entered the European nation over the holidays. This includes checks at Charles de Gaulle and Orly airports, as well as whatever limited information is available from train stations.