WASHINGTON — The Justice Department and a liberal Democratic senator have teamed up in rare alliance to restrict gun purchases by people whose names show up on watch lists as terror suspects.The bill, introduced late Thursday by Democratic Sen. Frank Lautenberg after two years of study produced an endorsement by the Justice Department, would give the attorney general power to block gun sales to terror suspects. In certain instances, the attorney general could let a sale go through; for example, when stopping the sale would hinder an investigation.

The measure also includes ways for potential gun purchasers to appeal the attorney general's denial.

"The administration finally realised that letting terrorists buy guns is dangerous," Lautenberg said. "This 'terror' gap in our gun laws has been open too long."

The Justice Department, which endorsed the idea in letters this week to leaders of the House of Representatives and the Senate, said the wait before approval of the law had been necessary in part to study potential situations where barring a gun purchase could interfere with investigations and intelligence collection.

"There are circumstances in which it may be counterproductive to mandate the denial of a firearm transfer to an individual on a terrorist watch list," said Justice Department spokesman Dean Boyd.

Letting a prospective gun buyer know that he is barred from the purchase because he is a terror suspect on a watch list could prompt the person to disappear and potentially hamper an investigation.

Suspects on federal watch lists can buy firearms if background checks come back clear of several prohibitions — such as felony convictions, mental illness and illegal immigration status.

Being on a terrorist watch list is not one of the flags that would prevent the gun sale, which law enforcement officials have cautioned could be exploited by would-be terrorists.