Credible intelligence from multiple sources indicates al-Qaeda is planning an attack on America in the coming months, US security chiefs have warned.
Ashcroft makes the case for heightened security and awareness
Attorney General John Ashcroft told a news conference that information showed al-Qaeda's specific intention was "to hit the United States hard".
Mr Ashcroft also named seven people who he said presented a clear danger.
He listed future events that could be targets; however, the US has not yet raised the current threat level.
Mr Ashcroft, together with FBI chief Robert Mueller, said there was intelligence about a plot, but this did not indicate the date, time, nor method of a possible attack.
Mr Mueller said 4 July celebrations, the Democratic and Republican party conventions, and the presidential elections in November could be at risk.
"The same events which fill most of us with hope and pride are seen by terrorists as possible targets for attack," he said.
They also released the names and photographs of seven suspects, including one woman, who could be part of a plot and asked for the public's help in tracking them down.
Some were US citizens or had spent a long time in the US.
"All present a clear and present danger to America. All should be considered armed and dangerous," Mr Ashcroft said.
Mr Ashcroft also warned that al-Qaeda may be changing its tactics, as members might travel with their families "to lower their profile" or be able to portray themselves as Europeans.
The attorney general said that the Madrid railway bombings, which came just before the Spanish general election, were believed by al-Qaeda members to have advanced their cause.
Political fallout from those attacks is believed to have contributed to the defeat of the governing party.
Mr Ashcroft warned al-Qaeda might try the same tactics in advance of the US poll.
His briefing came shortly after the US Energy Secretary, Spencer Abraham, announced a multi-million dollar initiative to curb the possibility of terrorists employing so-called dirty bombs.
These are devices that use conventional explosives to spread low-level radioactive material.
Despite the latest warnings, the Bush administration has no immediate plans to raise the colour-coded terror alert level.
The US has a five-colour system of threat alert running from "green" for "low risk", to "red" for "severe risk" of terrorist attack.
The level is currently set at "yellow", the midway point, which indicates an "elevated" risk of attack.
Some union leaders, who support the Democratic challenger to President Bush, John Kerry, have questioned the timing of Wednesday's announcement, saying it is suspicious in an election year, at a time when Mr Bush's opinion poll ratings are falling.
The suspects have all been sought for months
And Mr Kerry himself called on the Bush administration to do more than just issue warnings about possible terrorist threats.
"We deserve a president of the United States who doesn't
make homeland security a photo opportunity and the rhetoric of
a campaign," Mr Kerry said during campaigning in Seattle.
"We deserve a president who makes America safer."
White House spokesman Scott McClellan denied they were overplaying the threat, pointing to a "stream of credible intelligence" over the last couple of months.